Riot police surrounded the rally but no clashes were reported.
The main opposition Bangladesh Nationalist Party and its Islamist ally, Jamaat-e-Islami, are demanding the restoration of a constitutional provision that required incumbent governments to transfer power to a neutral caretaker administration to conduct polls.
Prime Minister Sheikh Hasina's government scrapped the 15-year-old system last year.
The opposition, led by former Prime Minister Khaleda Zia, says elections due in 2014 will be rigged if held under the current government without a caretaker system in place.
Zia said her alliance will stage a nationwide general strike on March 29 and will escalate its protests if the system is not restored by June 11.
"We will announce a series of protests across the country if the government fails to accept our demand," Zia told the cheering crowd.
General strikes are a common opposition tactic in Bangladesh to highlight demands and embarrass the government.
Bangladesh, a fledging democracy, has a history of political unrest. Zia and her archrival Hasina have alternated as prime minister since the country's last military ruler, Hussain Mohammad Ershad, was ousted in 1990.
