By Raushan Nurshayeva

ASTANA (Reuters) - Ten people were killed when sacked oil workers clashed with riot police in Kazakhstan on Friday, a rare violent protest in the tightly controlled Central Asian state that overshadowed celebrations to mark 20 years of independence.

Several people were also wounded after protesters stormed a stage in the oil city of Zhanaozen and set fire to the city administration building and the headquarters of the local unit of London-listed oil firm KazMunaiGas Exploration Production.

"Ten people were killed as a result of mass disorder. There are also some wounded, including police officers," Kazakh Prosecutor-General Askhat Daulbayev told a news conference, citing preliminary data.

He did not say who had been killed or how.

He said a group of investigators led by the interior minister had flown to the city on the orders of President Nursultan Nazarbayev "to take all necessary measures to preclude further criminal acts, identify and punish organizers of the disorder and restore public security in the town."

Violent protests scarcely occur in Kazakhstan, Central Asia's largest economy and oil producer, where 71-year-old Nazarbayev has ruled with a firm hand for more than 20 years. He has overseen massive foreign investment, mainly in oil and gas.

In a speech on Thursday, Nazarbayev stressed the need to preserve stability in the country of 16.6 million people, which has also witnessed an unprecedented spate of attacks by Islamist militants this year.

The clashes in Zhanaozen, a city of 90,000 people about 150 km (95 miles) inland from the Caspian Sea, marred celebrations to mark the 20th anniversary of Kazakhstan's independence from the Soviet Union.

A triumphal arch was unveiled in the futuristic capital Astana as part of lavish celebrations held nationwide. But in Zhanaozen, protesters -- some wearing red-and-blue overalls with a KazMunaiGas logo -- stormed a stage in the central square.

Video footage broadcast by privately owned television channel K+, and later on state television, showed a crowd of men breaking through barriers and toppling speakers from the stage.

"The lawbreakers attacked policemen, toppled the New Year tree, destroyed yurts (tents) and a scene laid out for the holiday and set a police bus on fire," Daulbayev said.

"As a result of the mass disorder, the buildings of the town administration, a hotel and the administration building of Uzenmunaigas were torched. Property of private persons and companies was also destroyed. Cars were burned and ATMs plundered."

Uzenmunaigas is the local unit of KazMunaiGas Exploration Production (KMG EP). The company said in a statement that its oil-producing facilities were working as normal, but that it had stepped up security.

KazMunaiGas EP, a unit of state oil and gas company KazMunaiGas, was hit this year by a three-month strike at its Uzenmunaigas unit near the city.

A total of 989 Uzenmunaigas workers were sacked and the company has said it will fall 8.5 percent short of its full-year production target for 2011.

"OUTRAGED" WORKERS

A local government worker said protesters had destroyed the traditional felt tents, known as yurts, near the concert stage.

"They were outraged that the administration was preparing a holiday for the town's residents ... and they started pelting stones at people walking by," Zhanna Oishibayeva, an aide to the governor of the local Mangistau region, said by telephone.

The short clip of video footage showed a few men chasing a policeman away from the stage. Police cars were seen driving fast through the crowd and one policeman fired pistol shots into the air while standing in a group of officers facing a crowd.

Social networking websites said that some local people had heard shooting in Zhanaozen. These claims could not be verified independently and staff at the local central hospital declined to comment when contacted by Reuters.

Daulbayev, the prosecutor-general, said "a group of hooligans" attacked peaceful residents and smashed parked cars.

"Responding to demands by law-enforcement bodies to stop their unlawful actions, a group of hooligans attacked law-enforcers, attempting to seize their weapons. They were using firearms and cold steel," he said.

"The prosecutor-general's office warns the organizers of mass disorder about their accountability to the law and calls on them to stop their unlawful actions."

A local hotel receptionist said all mobile and Internet communication had been switched off in Zhanaozen. A spokeswoman for Kazakhtelecom, the country's main operator of digital services, said the firm had nothing to do with the outages.

(Additional reporting by Mariya Gordeyeva and Dmitry Solovyov in Almaty; Writing by Robin Paxton and Dmitry Solovyov; Editing by Mark Heinrich)