Tens of thousands of protesters have rallied in cities across Bangladesh for a third consecutive day, demanding the execution of a religious political leader who was sentenced to life in prison for war crimes committed during the country's 1971 war of independence.

The sentencing of Abdul Quader Mollah by a war crimes tribunal on Tuesday for charges including murder, rape and torture was the second verdict in trials that have reopened the wounds of Bangladesh's struggle to break away from Pakistan.

Mollah, 64, is the assistant secretary-general of Jamaat-e-Islami, the country's largest religious political party.

Public outrage was fuelled by bloggers and activists using Facebook and other social media websites who called for mass protests.

Protest leaders have called for activists around the country to converge on Dhaka on Friday for a mass rally.

Demonstrations were held for the third day running on Thursday in more than a dozen cities, including the capital Dhaka, Chittagong, Sylhet, Rajshahi, Rangpur and Khulna.

Thousands of people poured on to Dhaka's Shahbag Avenue, vowing to continue their protest until authorities agreed to their demand to execute Mollah. Police blocked nearby streets and diverted traffic.

"We won't go home until we are satisfied," said student Shams Islam as the crowd sang patriotic songs, beat drums and burned effigies of the Islamist leader.

"We are not only pushing for justice, but also reminding our political leaders that the nation can unite and stand together for a genuine cause," said another activist, Abdul Latif.

"Shahbag has become a model for unity, especially for the young generation aspiring for a happy and prosperous Bangladesh," he said.

Bangladesh became part of Pakistan at the end of British rule in 1947 but broke away in 1971 after a war between Bangladeshi nationalists, who were backed by India, and Pakistani forces that cost 3 million lives.

Some factions in what was then East Pakistan opposed the break with Pakistan, and numerous abuses were committed during the nine-month war. Jamaat denies accusations that it opposed independence and helped the Pakistani army.