The violence in Zhanaozen broke out as Kazakhstan celebrated its 20th anniversary as an independent country, Russian news agency RIA Novosti reported. Kazakhstan was part of the Soviet Union for decades.
Zhanaozen is in the oil-rich province of Mangystau. Local news media cited by Novosti said those involved in Friday's clashes were employees of KazMunaiGas, a state-controlled company where workers have been engaged in strikes for several months.
Prosecutor Ashkat Daulbayev said a "group of hooligans" attacked police. He said they set fire to an office of a KazMunaiGas subsidiary, and a hotel and several cars were also burned.
Local residents said authorities had cut off Internet and WiFi access.
Russian President Dmitry Medvedev and Prime Minister Vladimir Putin sent congratulatory telegrams Friday to Nursultan Nazarbayev, who has been president of Kazakhstan since 1991, and Prime Minister Karim Massimov.