AKTAU, Kazakhstan (Reuters) - An outspoken critic of Kazakh President Nursultan Nazarbayev was jailed for seven-and-a-half years on Monday for his part in a failed attempt to overthrow the government of the oil-producing Central Asian state.

Vladimir Kozlov, leader of the unofficial Alga! party, was found guilty of orchestrating dissent among striking oil workers in the prelude to riots last December that left 15 people dead and dented Kazakhstan's reputation for stability.

Kozlov, 52, denied the charges. Speaking at the end of the seven-week trial on October 1, he described his case as an "undisguised attempt" to stifle civic protest and labor rights.

In more than two decades as president, Nazarbayev, 72, has eschewed democratic freedoms in pursuit of the resource-fuelled growth and investment that has made Kazakhstan's economy the largest in Central Asia.

In recent years, authorities have tried to balance their desire to preserve stability and economic growth with efforts to improve the country's image on the world stage.

Prosecutors had earlier said Kozlov, whose property was also confiscated by the court, had acted under orders from Mukhtar Ablyazov, an arch-opponent of Nazarbayev and the former head of BTA bank who fled Kazakhstan in 2009.

Britain granted Ablyazov political asylum as he awaited embezzlement charges brought by the bank, which he has said were politically motivated. But his whereabouts are unknown since he fled after being convicted in February in Britain of contempt of court.

Two independent opposition activists standing trial alongside Kozlov, Serik Sapargaly and Akzhanat Aminov, were given suspended sentences of four and three years respectively.

On December 16, police opened fire on protesters, including hundreds of sacked oil workers, when riots broke out during a street party in the town of Zhanaozen to mark 20 years of Kazakhstan's independence from the Soviet Union.

At least 14 people were killed. Rioting spread the next day to the nearby village of Shetpe, where another person died.

(Reporting by Dmitry Solovyov; Writing by Robin Paxton; Editing by Mark Heinrich)