Riot police fired water cannon and tear gas on Thursday to break up a three-month protest against a vast copper mining project run by the powerful Myanmar military and its partner, a subsidiary of a Chinese arms manufacturer.
Activists said at least 50 people had been injured and 23 were in hospital, some of them suffering burns after incendiary devices were hurled into their camps by the police. Media described the devices as "phosphorous bombs".
After decades of oppression, the Monywa mine has become a test of Myanmar's commitment to reform as protesters explore new-found freedoms, including a relaxation of laws on protests that took effect in July.
It also illustrates growing resentment towards Chinese companies that have expanded in recent years across the country.
Witnesses said truckloads of police arrived at six camps near the mine in the Sagaing region in Myanmar's northwest, where thousands have demonstrated against a $1 billion expansion of the project and what they call the unlawful confiscation of more than 7,800 acres of land.
Shin Oattama, a Buddhist monk who had helped the villagers, told Reuters by telephone security forces began to use water cannon and other weapons at about 3 a.m.
"They shot some sort of canisters that caused fire at the camp. We just don't know what sort of weapon it was," he said. "We are now seeking refuge at a nearby village. There's no ambulance, no doctor to take care of the injured," he said.
Myo Thant, a member of the 88 Generation Students Group that long opposed military rule, who has been monitoring the situation in Monywa, said 22 monks and one other person were in hospital with injuries sustained during the police action.
He said police had turned water cannon on the protesters and then some officers had used "these strange weapons".
"The stuff from these canisters got caught on the clothes and bodies of the victims. When they shook their robes to remove this stuff, fire started," he said.
But Zaw Htay, a spokesman for President Thein Sein, told Reuters the police had used only water cannon, tear gas and smoke bombs to disperse the protesters.
"No chemical weapons were used by the police to do their duty," he said.
Land disputes are a growing problem in Myanmar. Protests were suppressed under a military junta in place until last year but have become more common as Thein Sein opens up the country, also known as Burma, and pushes through reforms including the right to strike.
Some activists say much remains to be done.
"This is an example of the skin-deep nature of Burma's reforms," said Mark Farmaner of London-based advocacy group Burma Campaign UK. "The new right-to-protest law was hailed as a major reform but it is clear there is still no right to protest in Burma."
Aung San Suu Kyi, Nobel Peace laureate and a member of parliament, was going ahead with a visit to the site on Thursday, her National League for Democracy (NLD) party said. She intended to speak to the protesters about their grievances.
NLD official Ohn Kyaing told Reuters by telephone she had flown to the central city of Mandalay and was going on to Monywa by road.
The copper mine, Myanmar's biggest, is run by a unit of China North Industries Corp, a prominent weapons manufacturer, under a deal signed in June 2010 after Canada's Ivanhoe Mines Ltd pulled out in 2007.
It is backed by the military-owned Union of Myanmar Economic Holdings Ltd, which operated extensively under the military regime that ruled for almost half a century until 2011.
The Global Times, an influential tabloid published by China's Communist Party mouthpiece, the People's Daily, said in an editorial on Thursday it would be a "lose-lose situation for China and Myanmar if the project is halted".
"Only third parties, including some Western forces, will be glad to see this result," it said, blaming "some Westerners" and non-government organizations for instigating the protests.
"We must not give up on the project. Even if it is eventually stopped, Chinese companies should receive compensation according to the contract and international practice," it said.
As the number of land disputes increase, villagers appear emboldened by reforms under Thein Sein, who took office in March 2011, and are pushing back.
Authorities warned the protesters late on Tuesday to clear the site by midnight that day so that a parliamentary commission could carry out an investigation, but the camps remained.
State television said all project work had been halted since November 18 because of the protests.
Protests stretching back at least three months have involved thousands of villagers and supporters. They told Reuters in September that four of 26 villages at the project site had already been displaced, along with monasteries and schools.
(Additional reporting by Ben Blanchard in Beijing and Andrew R.C. Marshall in Bangkok; Writing by Alan Raybould and Jason Szep; Editing by Paul Tait and Robert Birsel)