The protesters in Dhaka urged Prime Minister Sheikh Hasina to review a verdict sentencing a senior leader of Jamaat-e-Islami to life in prison for killings and other crimes.
The protesters said the life term was not enough as Abdul Quader Mollah was found by a tribunal guilty of five charges, including playing a role in the killing of 381 unarmed civilians.
The government will appeal the sentence. A defence lawyer said they will also appeal seeking an acquittal for Mollah, whose verdict is the second after Hasina came to power through a 2008 election and formed a tribunal to try those suspected of war crimes during the war.
Both sides have 30 days to appeal to the Supreme Court.
The life sentence comes after a former party member was sentenced to death last month.
The exact number of the protesters was difficult to know but streets near Dhaka University were filled with 1971 fighters, students, political activists, teachers and people from various walks of life. Some organizers put the number at up to 200,000, and Anjan Roy, a television talk show moderator who lost more than a dozen family members and relatives in 1971, told Associated Press more than 100,000 joined the rally.