More than 100 other people, including many police officers, were injured.

The incident took place in the state of Oaxaca, where two high-profile union leaders were arrested last week, accused of corruption.

Members of the CNTE union, which has a history of radical activism, have been blocking roads in southern Mexico ever since the union leaders were arrested.

The authorities said the police were trying to clear the roads when unknown gunmen began firing at both sides in order to create chaos and conflict.

Mexico's National Security Commission originally said in a statement that the police officers involved in the operation near the town of Nochixtlan were not carrying guns,

But federal police chief Enrique Galindo later said that an armed unit was deployed after shots were fired at the police and the protesters by "unidentified people" not linked to the demonstration.

The state-owned oil company, Pemex, warned on Friday that it might be forced to close a refinery in the area if the highway linking Oaxaca to the capital, Mexico City, remained blocked.

The dissident CNTE union opposes education reforms introduced by President Enrique Pena Nieto in 2013.

The changes include new measures to assess teachers' performance.

Union members say the corruption allegations against their leaders are politically motivated.

Ruben Nunez was accused of siphoning off money raised illegally from union members. Prosecutors also accused him of money laundering.

"He raised more than 24m pesos ($1.3m; £900,000) illegally between 2013 and 2015," said prosecutor Gilberto Higuera.

For two years the union demanded a 3.5% cut of purchases made by unionised members from some businesses in Oaxaca, according to Mr Higuera.

The money was taken from the education workers' payslips, which prosecutors say was controlled by the union.

Mr Nunez's deputy, Francisco Villalobos, was arrested for allegedly stealing textbooks.

It estimates that 65.3m people were either refugees, asylum seekers or internally displaced at the end of 2015, an increase of 5m in a year.

This represents one in every 113 people on the planet, the UN agency says.

Meanwhile, the UN refugee chief says a worrying "climate of xenophobia" has taken hold in Europe as it struggles to cope with the migrant crisis.

The influx of people, the biggest since World War Two, has led to greater support for far-right groups and controversial anti-immigration policies.

In its annual report marking World Refugee Day, the UN said it was the first time the number of refugees worldwide had passed the 60m mark.

Over half of the total comes from just three countries: Syria, Afghanistan and Somalia.

Are more people on the move?

Migrant crises through history

Despite the huge focus on Europe's migrant crisis, the UN said 86% of the world's refugees were being sheltered in low and middle income countries.

Turkey is the biggest host country for refugees worldwide, with 2.5m people, followed by Pakistan and Lebanon.

More than 1,011,700 migrants arrived in Europe by sea last year, according to theInternational Organisation for Migration (IOM), although other agencies put that number much higher.

Some 35,000 arrived by land, the IOM said.

The preferred destinations for most of them were richer northern countries like Germany and Sweden.

This is reflected in the UN's figures for new asylum applications in 2015, which show that Germany was the largest single recipient, followed by the United States and Sweden.

Most Syrian refugees in Turkey are covered by the Turkish government's temporary protection scheme so do not count as asylum claimants.

The European migration crisis has caused significant political rifts within the EU, with some states inside the border-free Schengen area putting up fences and re-imposing frontier controls.

The European bloc reached an agreement with Turkey in an attempt to limit the mass movement of people into the EU, a deal that has been heavily criticised by human rights groups.

In separate remarks, the UN refugee chief said European leaders needed to do more to coordinate policies and to combat negative stereotypes about refugees.

"Those who do the opposite, who stir up public opinion against refugees and migrants, have a responsibility in creating a climate of xenophobia that is very worrying in today's Europe," Filippo Grandi told AFP news agency.

He said it was unfortunate that some decisions taken by the EU to handle the crisis "were not implemented", calling it "a missed opportunity".

Reynaldo Gonzalez is accusing Google, Facebook and Twitter of providing "material support" to extremists.

He alleges they "knowingly permitted" the so-called Islamic State group - referred to as "ISIS" in his legal action - to recruit, raise money and spread "extremist propaganda".

The companies said they had policies against extremist material.

Mr Gonzalez's daughter Nohemi was among the 130 people killed when extremists attacked Paris' Bataclan concert hall, bars, restaurants and the national football stadium in nearby Saint-Denis.

"For years, [the companies] have knowingly permitted the terrorist group ISIS to use their social networks as a tool for spreading extremist propaganda, raising funds and attracting new recruits," court papers filed in California on Tuesday read.

"This material support has been instrumental to the rise of ISIS, and has enabled it to carry out numerous terrorist attacks, including the 13 November 2015 attacks in Paris, where more than 125 were killed, including Nohemi Gonzalez."

Mr Gonzalez alleged that, without Twitter, Facebook, and the Google-owned YouTube, the "explosive growth of ISIS over the last few years into the most-feared terrorist group in the world would not have been possible" because they had provided the infrastructure necessary for the group to get its message out.

He quoted Brookings Institution research as saying Islamic State had "exploited social media, most notoriously Twitter, to send its propaganda and messaging out to the world and to draw in people vulnerable to radicalisation".

In statements given to the Associated Press news agency, Facebook and Twitter said the case was without merit, and all three companies cited their policies against extremist material.

Twitter, for example, said it had "teams around the world actively investigating reports of rule violations, identifying violating conduct, and working with law-enforcement entities when appropriate".

Facebook's statement said if the company saw "evidence of a threat of imminent harm or a terror attack", it would contact law enforcement.

Google said it would not comment on pending legal action, but noted that it had "clear policies prohibiting terrorist recruitment and content intending to incite violence and quickly removed videos violating these policies when flagged by our users".

Under US law, internet companies are generally exempt from liability for the material users post on their networks.

Section 230 of the 1996 Communications Decency Act provides a legal "safe harbour" for companies such as Twitter and Facebook, stating that "no provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider".

But it is unclear whether that legal defence would suffice in this case.

Ari Kresch, one of the lawyers representing Mr Gonzalez, told AP: "This complaint is not about what ISIS's messages say… it is about Google, Twitter, and Facebook allowing ISIS to use their social media networks for recruitment and operations."

The legal action also alleges that YouTube shared revenue with the terror group from adverts that ran with its videos.

Benjamin Wittes, a senior fellow at the Brookings Institution, agreed that the legal "safe harbour" might not shelter social-media companies in such cases.

But he told AP Twitter the causal link between the alleged support for extremists and the attack was very weak.

Jo Cox, 41, Labour MP for Batley and Spen, was left bleeding on the ground after the attack in West Yorkshire. A 52-year-old man was arrested nearby.

Her husband Brendan Cox said she would want people "to unite to fight against the hatred that killed her".

Tributes flooded in from politicians including David Cameron, Jeremy Corbyn and US Secretary of State John Kerry.

Vote Leave and Remain have both suspended campaigning in the EU referendum in light of the attack.

Mrs Cox is the first sitting MP to be killed since 1990, when Ian Gow was the last in a string of politicians to die at the hands of Northern Irish terror groups.

Labour leader Mr Corbyn said the country would be "in shock at the horrific murder", describing the MP as a "much loved colleague".

He added: "Jo died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve.

"In the coming days, there will be questions to answer about how and why she died.

"But for now all our thoughts are with Jo's husband Brendan and their two young children. They will grow up without their mum, but can be immensely proud of what she did, what she achieved and what she stood for."

Cafe owner Clarke Rothwell, who witnessed the attack, said he heard a "loud popping noise that sounded like a balloon burst - a loud balloon".

"When I looked round there's a man stood there in his 50s with a white baseball cap on and a jacket with a gun, an old fashioned looking gun in his hand," he said.

"He shot this lady once and then he shot her again, he fell to the floor, leant over shot her once more in the face area.

"Somebody tried to grab him, wrestling with him and then he wielded a knife, like a hunting knife, just started lunging at her with a knife half a dozen times. People were screaming and running from the area".

Eyewitness Hithem Ben Abdallah, said the mother of two was left lying and bleeding on the pavement after the incident.

Mr Abdallah, 56, was in a cafe next door to the library shortly after 13:00 BST when he heard screaming and went outside.

"There was a guy who was being very brave and another guy with a white baseball cap who he was trying to control and the man in the baseball cap suddenly pulled a gun from his bag".

After a brief scuffle, he said the man stepped back and the MP became involved.

Mr Abdallah said the weapon had "looked handmade" and a man who had been wrestling with the gunman continued even after seeing the gun.

He said: "The man stepped back with the gun and fired it and then he fired a second shot, as he was firing he was looking down at the ground."

"He was kicking her as she was lying on the floor", he said.

Her husband, Brendan, has since tweeted a picture of his wife standing by the side of the River Thames in London.

David Cameron also tweeted to express his condolences.

Mr Kerry said: "It is an assault on everybody who cares about and has faith in democracy."

Leeds North East MP Fabian Hamilton said he had known Jo Cox since before she became an MP.

Mr Hamilton said: "And I'm just completely devastated, I know Birstall reasonably well, and I think we're all totally shocked."

Huddersfield MP Barry Sheerman said he was "absolutely stunned".

"I was a mentor to her," he said.

"She was the same age as my daughter. She was one of the real talents in parliament, people loved her in the constituency."

Ms Cox, who was born in Batley, was elected in 2015.

She was educated at Heckmondwike Grammar School and graduated from Cambridge University in 1995.

A former head of policy for Oxfam, she also worked as an adviser to Sarah Brown and Baroness Kinnock.

Mark Goldring, Oxfam's chief executive, said: " Oxfam is deeply shocked to hear the news. Our thoughts and sympathies are with Jo and her family at this difficult time."

Most MPs hold surgeries in their constituency to give people an opportunity to meet them and discuss matters of concern.

They are usually held once a week and advertised locally or online. An MP may take up an issue on a constituent's behalf.