DHAKA (AFP) - Riots broke out in several Bangladesh cities on Tuesday after a court sentenced a senior Islamist opposition official to life in prison for mass murder during the 1971 liberation war against Pakistan.

Abdul Quader Molla, 64, the fourth-highest leader of the Jamaat-e-Islami party, was the first politician to be found guilty by the International Crimes Tribunal, a much-criticised domestic court based in Dhaka.

Molla cried "Allahu Akbar!" (God is greater) and said the charges, which also include crimes against humanity, were false after presiding judge Obaidul Hassan delivered the verdict in a crowded and tightly guarded court.

"He deserved death sentence because of the gravity of the crimes. But the court gave him life imprisonment," said Attorney General Mahbubey Alam, adding Molla was found guilty of five out of six charges including mass murder.

The judgement sparked immediate protests by Jamaat, the country's largest Islamic party which enforced a nationwide strike in anticipation of the conviction.

It warned it would resist "at any cost a government blueprint" to execute its leaders.

Police said they had clashed with protesters in the capital Dhaka and in several other cities across the country in the aftermath of the verdict.

In northwestern Rajshahi, police fired tear gas and rubber bullets at about 500 Jamaat supporters who hurled at least a dozen homemade firebombs.

The verdict is the second by the tribunal. On January 21, a top TV preacher who is also an ex-Jamaat official was sentenced to death in absentia for murder and genocide.

Ten other opposition figures - including the entire leadership of Jamaat and two from the main opposition Bangladesh Nationalist Party (BNP) - stand accused of war crimes.

Both Jamaat and BNP have labelled the cases "show trials" aimed at barring the leaders from upcoming polls. International rights groups have questioned the proceedings.

Prosecutors said that during the war, Molla was a senior figure in the then-student wing of Jamaat, while studying physics at Dhaka University, and was a member of a pro-Pakistan militia accused of killing hundreds of thousands of people.