Bangladesh's political parties launched campaigns on Monday ahead of general elections next month, with opposition groups boycotting the vote after a surge of arrests.

Opposition parties say thousands from their ranks have been detained after huge protests accusing Prime Minister Sheikh Hasina of ruling with an iron fist and demanding her resignation.

Elections are set for January 7.

"I am 100 percent confident that the victory will be mine," R. A. M. Obaidul Muktadir Chowdhury, a ruling Awami League candidate from the eastern district of Brahmanbaria, told AFP.

Kayser Kamal, legal chief for the main opposition Bangladesh Nationalist Party (BNP), said at least 21,835 members had been arrested since October 25.

BNP chief Khaleda Zia, Hasina's arch-rival and a two-time former prime minister, has been living under effective house arrest since her release from a 17-year prison sentence in 2020.

On Monday, a Dhaka court denied bail to BNP's de facto chief Mirza Fakhrul Islam Alamgir.

"It is part of a political scheme so that no opposition leader or activist can raise their voice," Kamal said.

Prothom Alo, a leading Bengali daily, reported on Monday that 1,249 opposition activists have been sentenced to different terms since September, mostly on charges of protest violence, arson and obstructing the police.

Bangladesh's highest court upheld last month a ban on Jamaat-e-Islami, the country's largest Islamist party, from taking part in polls.

The party says more than 3,100 members have been arrested since October 25.