At least 10 people have been killed as striking oil workers clashed with government forces in a town in western Kazakhstan, officials say.
Eyewitnesses said police fired on unarmed protesters in the town of Zhanaozen, but the authorities say they were forced to defend themselves.
The clashes occurred when police tried to clear the town square, occupied by the workers for more than six months.
They are demanding better pay, but their action has been declared illegal.
The clashes came as Kazakhstan marked the 20th anniversary of independence from the Soviet Union, with celebrations throughout the country.
Correspondents say the unrest will come as a shock to President Nursultan Nazarbayev, who has ruled the republic since before independence and kept a tight rein on public protest.
There were signs that the authorities were trying to control media reports, they add, with internet users reporting difficulties accessing independent news websites and Twitter.
Offices on fire
Police moved in to clear the square early on Friday morning for independence celebrations but were charged by the oil workers.
An eyewitness told the BBC police had opened fire on the unarmed protesters and one man died in her arms from a shot to the chest.
A number of buildings including local government offices, a hotel and the office of the state oil company were set on fire, Kazakhstan's General Prosecutor Askhat Daulbayev said.
Preliminary information indicated 10 deaths, with police officers among the injured, he added.
"Civilians, who had gathered in the main square to celebrate the 20th anniversary of the country's independence, were attacked by a group of hooligans," he said.
Some yurts and a portable stage erected for Independence Day were destroyed, the prosecutor said.
The Kazakh opposition TV channel K-Plus showed what appeared to be the beginning of the unrest, with oil workers running on to the stage, tipping over speakers and pushing officials before police arrived.
Mr Daulbayev added that a criminal investigation had been opened into the disturbances, which had resulted from "criminal actions".
A team of investigators led by Interior Minister Kalmukhanbet Kasymov had flown to the town to "identify and punish the organisers", he said.
Vladimir Kozlov, leader of an unregistered opposition party, said the longevity of the protest had annoyed the authorities.
"The fact that the people have been standing on the square for seven months and not leaving irritates them," he told K-Plus.
"They haven't used DDT [pesticide] yet, but they have tried all other means. They are still standing. This irritates the authorities the most."
Oil workers at the Ozenmunaigas oil field, which employs the Zhanaozen workers, have higher than average salaries, but strike organisers say the workers are owed danger money for the tough conditions they work in.
The oil-rich but remote western Mangistau region has seen several cases of industrial action this year, with oil fields responding by sacking hundreds of workers.