The trial of the most vocal opposition leader in Kazakhstan on charges of seeking to overthrow the government began Thursday, with critics calling the case an attempt by the country's strongman president to silence his opponents.

Details of the charges against Vladimir Kozlov were not made public ahead of the trial, but appear to be related to violence in the eastern town of Zhanozen in December. At least 14 people died after police opened fire on striking oil workers.

Kozlov, 52, heads the unregistered political party Alga, which has been unwavering in its criticism of President Nursultan Nazarbayev, who has led Kazakhstan since it gained independence after the 1991 collapse of the Soviet Union.

Nazarbayev thoroughly dominates Kazakhstan's political life and was re-elected last year with 95 percent of the vote. The election was sharply criticized by international observers for ballot-stuffing, voter intimidation and a lack of transparency.

Alga says the trial is aimed at eliminating opposition to Nazarbayev's regime. Kozlov has also been charged with inciting social unrest and heading a criminal gang and faces up to 13 years in jail. Two other opposition activists are also standing trial on similar charges.

Thirteen people from Zhanaozen have been jailed for direct involvement in the unrest that broke out on Dec. 16. The unrest came after a seven-month long occupation of the main square by oil workers demanding higher salaries.

The events stunned the country and exposed seething social tensions in the otherwise stable Central Asian nation bordering China and Russia, prompting international concern. Kazakhstan, a vast nation with copious oil and gas reserves, is seen as a reliable partner by the West and cooperates closely on security and energy, but has come under withering criticism from rights groups for its weak democratic credentials.

The U.S. State Department's 2011 country report on human rights practices in Kazakhstan lamented significant restrictions on freedom of assembly and association. The U.S. will closely follow Kozlov's trial, Assistant Secretary of State Robert Blake said Wednesday, calling on Kazakh authorities to conduct the trial in a "fair, impartial and open way."