user: Article: 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; Â£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.
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Summarize: Six people have been killed in southern Mexico in clashes between protesting teachers and police.


Article: According to a Sunday Times investigation, Mark Bonar - a London-based private doctor - charges sports stars thousands of pounds for drug programmes.
The newspaper says it has secretly filmed the medic claiming that he has prescribed performance-enhancing drugs such as EPO, steroids and human growth hormone to 150 elite sports professionals from the UK and abroad over the past six years, including:
There is no suggestion that any Premier League clubs were aware of any alleged wrongdoing and the Sunday Times says it has no independent evidence Bonar treated the players.
Bonar has told the Sunday Times he treated the athletes for medical reasons and not to enhance their performance. There is no suggestion the substances were illegal.
Athletes contacted by the newspaper either denied Bonar had treated them or declined to comment.
The government investigation stems from claims made by a whistleblower athlete who was banned for breaching anti-doping rules in 2014.
According to the newspaper, the sportsman - who wishes to remain anonymous - approached Ukad with evidence that indicated Bonar had allegedly prescribed him performance-enhancing drugs.
Ukad says it did begin an investigation into Bonar but found that he was outside of its jurisdiction as he was not governed by a sport, and decided not to pass the case to the General Medical Council (GMC) or to contact him.
The Sunday Times - in collaboration with an aspiring athlete - then secretly recorded Bonar allegedly prescribing a series of prohibited drugs to the runner. It is claimed that Bonar then went on to talk about the other sportspeople he says he treated.
Bonar does not currently hold a licence and is unable to practise medicine in the UK, according to the GMC.
He is also facing a misconduct tribunal later this month. It is unrelated to allegations of doping.
Ukad told the BBC it would conduct an independent review into the case.
"I am shocked and deeply concerned by these allegations," Culture, Media and Sport Secretary John Whittingdale said in a statement.
"I have asked for there to be an urgent independent investigation into what action was taken when these allegations were first received and what more needs to be done to ensure that British sport remains clean.
"There is no room for complacency in the fight against doping and the government is already looking at whether existing legislation in this area goes far enough. If it becomes clear that stronger criminal sanctions are needed then we will not hesitate to act."
In response to the Sunday Times article, Ukad chief executive Nicole Sapstead said the organisation was "deeply concerned and shocked by the allegations".
Ukad's statement also said it had:
World Anti-Doping Agency president Craig Reedie told the BBC it was "pretty dreadful news" and backed calls for an inquiry, but added: "It is best to wait for Ukad to do their own investigation, then we have full knowledge of the facts to work with.
"My own guess is if they had sufficient evidence then they would have acted."
Ukad, which receives around £7m funding each year, has been widely praised for its work in the fight against cheating in sport.
It has been helping Russia's anti-doping agency run testing programmes after Russia was banned from all athletics competitions following accusations of widespread doping. And it was recently appointed as the secretariat for an IOC taskforce that will oversee testing procedures at the Olympics.
Last year a study commissioned by Uefa suggested 7.7% of players in a sample of 879 had high testosterone levels, although European football's governing body said the figures "didn't present evidence of potential doping".
Earlier this year Sapstead told the BBC that football was "at risk" of doping problems.
"When you're looking at a sport like football that commands the sort of salaries that the players can command, its fanbase, its ticket sales, its broadcasting rights - if that isn't a risk then I don't know what is, notwithstanding the physical demands of the sport itself," she said.
"Add that into a nice big mix and you've got everything pointing to a doping issue."
In response, the Football Association said Sapstead's comments were "speculative" and "unhelpful", adding that they had no "evidential basis".
A recent BBC Sport investigation learned that only eight drugs tests were conducted in Scottish football over a nine-month period between April and December 2015. There were 1,583 tests in English football over the same period.
Former Partick Thistle player Jordan McMillan is the only British footballer currently banned by Ukad, for taking cocaine.
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Summarize:

assistant: The government has ordered an inquiry into the UK Anti-Doping Agency (Ukad) over its handling of an investigation into alleged doping activities by a doctor involving Premier League footballers and other leading athletes.