More than 200 people were killed in street clashes between the security forces and opposition supporters before and during the January 5 polls.
Opposition parties say another 300 of their supporters were abducted ahead of the vote, which they boycotted, leaving the ruling Awami League to stand uncontested.
Hundreds of people took part in Wednesday's protest in the capital Dhaka, many carrying photographs of relatives who have been missing since the bloody crackdown.
"They were never returned," she said. "Our plea to the government and the RAB: please return them. They did no harm."
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Syeda Shammi Sultana said she had not seen her husband since plain-clothes security forces detained him in Dhaka.
"My husband's only crime was that he was a BNP supporter," she said, referring to the main opposition Bangladesh Nationalist Party (BNP).
Rights groups said the disappearances reached levels not seen since Bangladesh won independence in 1971.
"We've investigated scores of such disappearance cases and found that in most cases security forces were involved."
The government has denied that its agencies were behind the disappearances of political opponents.
RAB spokesman Mufti Mahmud told AFP the force was "not involved in any such abductions or disappearances".
The United States was among a host of countries to demand new polls that "credibly express the will" of the people after the Awami League romped to victory in the January ballot.
Bangladesh's military has a long history of intervening in politics and the former territory of East Pakistan has seen more than a dozen coups since its bloody war of secession from Islamabad more than 40 years ago.