Tens of thousands of Bangladeshi opposition supporters defied monsoon rains to join a huge protest in the capital Dhaka on Wednesday to demand the prime minister's resignation ahead of next year's elections.

The Bangladesh Nationalist Party (BNP) says its members have been subject to a renewed crackdown and has accused the ruling Awami League of planning to rig the polls, due by January.

It is demanding that Prime Minister Sheikh Hasina step down and allow the appointment of a caretaker government to ensure the vote is conducted fairly.

"This government is anti-Bangladesh. This government is anti-democracy," Mirza Fakhrul Islam Alamgir, the BNP's secretary-general, told a crowd massed outside the party's headquarters.

Alamgir said around 1,000 BNP supporters had been detained in the past month as part of a renewed nationwide crackdown against the opposition.

Amanullah Aman, a charismatic former student leader who played a key role in the ouster of a previous military junta in 1990, told the rally that the opposition "won't go back home without restoring democracy".

Around 50,000 people attended the rally, which dispersed without incident, a senior officer in the Dhaka Metropolitan Police told AFP on condition of anonymity.

Western governments have expressed concern over the political climate in Bangladesh, where Hasina's party dominates the legislature and runs it virtually as a rubber stamp.

Her security forces are accused of detaining tens of thousands of opposition activists, killing hundreds in extrajudicial encounters and disappearing hundreds of leaders and supporters.

The elite Rapid Action Battalion (RAB) security force and seven of its senior officers were sanctioned by Washington in 2021 in response to those alleged rights abuses.

Washington has called for a free and fair election and two senior State Department representatives are meeting with local officials in Dhaka this week.

AFP