Carrying replicas of boats - the traditional election symbol of the main opposition Awami League party - and chanting slogans such as "No reforms, no polls" and "Accept reforms before it is too late", the demonstrators marched from Gabtali, about 20km west of central Dhaka.
The marchers burnt the chief election commissioner, MA Aziz, in effigy.
Opposition parties have accused Aziz of being too partisan and said they would not take part in any elections with him at the helm.
The march was on the sixth and final day of protests, which were organised by Bangladesh's 14-party opposition alliance before parliamentary elections next January.
The opposition alliance led by Sheikh Hasina, the former prime minister and leader of Awami, reaffirmed that it would "boycott and resist" the parliamentary election unless the government carried out electoral reforms.
"The massive turnout in the series of marches since July 25 demonstrates people's strong support for the reforms needed to make the election free and fair," the Awami general secretary, Abdul Jalil, said on Sunday.
"We will keep adding pressure on the government until it bows to popular wishes."
Begum Khaleda Zia, the Bangladeshi prime minister, accused the opposition of trying to foil the election and disrupt democracy.
The ruling Bangladesh Nationalist party also held a march on Sunday, the second day of a three-day programme designed to counter the opposition show of strength.
More than 5,000 police and security forces were deployed along the route "to prevent any violence or any subversive acts", said Kohinur Mia, the deputy commissioner of Dhaka police.
A series of nationwide bombings linked to a radical Islamic group has plagued Bangladesh since August last year. The attacks killed 28 people, including four suicide bombers.
In the last two years serious political violence has also rocked the country. A former finance minister and a popular opposition member of parliament have been killed.