A war crimes court has found Islamist Ghulam Azam guilty of five charges relating to Bangladesh's 1971 war of independence with Pakistan.
Ghulam Azam was sentenced to 90 years in jail for his involvement in mass killings and rape during the war.
Supporters of Bangladesh's main Islamist Jamaat-e-Islami party, which he led from 1969 until 2000, clashed with police ahead of the verdict.
It is the fifth sentence passed against current and former party leaders.
The court found Mr Azam, 90, guilty of five charges including conspiracy, incitement, planning, abetting and failing to prevent murder.
He faced more than 60 counts of crimes against humanity for his role in setting up militia groups which carried out atrocities during the war.
Mr Azam has denied the charges, which his supporters say were politically motivated.
The prosecution had been seeking the death penalty.
But the three-judge panel said that while Mr Azam deserved capital punishment, he received a prison term because of his advanced age.
Spiritual leader
The mood in Bangladesh is tense, with police on all major streets of the capital and security beefed up around the country, the BBC's Mahfuz Sadique in Dhaka says.
Before the verdict was announce, police reportedly fired rubber bullets to disperse Jamaat-e-Islam supporters protesting in Dhaka and several other cities on Monday.
Journalists were among a number of people hurt in the violence in the Dhalpur district of Dhaka, police say.
On the eve of the verdict there were sporadic clashes in different parts of the capital with reports of some injuries, he adds.
Previous verdicts for former Jamaat leaders have led to deadly protests involving party supporters.
More than 100 people have been killed since January in political violence sparked by verdicts handed down by the International Crimes Tribunal.
Pro-government groups have also taken to the streets demanding death sentences for those being tried, accusing the tribunal of being too lenient.
In February, thousands staged vigils in Dhaka demanding the death penalty for Abdul Kader Mullah, who was sentenced to life for crimes against humanity.
Jamaat leader Delwar Hossain Sayeedi and the party's assistant secretary-general, Muhammad Kamaruzzaman, were both indicted and sentenced to death by the tribunal earlier this year.
The International Crimes Tribunal in Bangladesh was set up by the current Awami League-led government in 2010 to try alleged collaborators of the Pakistani army during Bangladesh's war of independence.
Human rights groups have said the tribunal falls short of international standards.
Mr Azam's defence lawyers say the charges are based only on newspaper reports of Mr Azam's speeches at the time, and none have been proved.
Jamaat has called for a general strike in protest.
Mr Azam was the party's leader from 1969 until 2000 and is seen by many as its spiritual leader.
Described by party colleagues as a writer and Islamic thinker, Mr Azam was strongly opposed to Bangladesh's independence from Pakistan, arguing at the time that it would divide the Muslim community.
The exact number of people killed during Bangladesh's nine-month war of secession is unclear: official Bangladeshi figures suggest as many as three million people died, but independent researchers suggest the death toll was around 500,000.
