As well as the thousands arrested – many from the key Bangladesh Nationalist Party (BNP) – a staggering number also face charges.
According to the BNP, about half of its five million members “face politically motivated prosecution”, Human Rights Watch said.
“The arrests, they are not leaving anyone behind, from senior level to the ground level,” one activist told the non-governmental rights advocacy group.
Prisons are now at more than double their capacity, the group said.
The South Asian nation of around 170 million people holds a general election on January 7, with Prime Minister Sheikh Hasina eyeing her fourth consecutive term in power.
‘Do or die’: strikes, clashes roil Bangladesh as opposition vows poll boycott
‘Do or die’: strikes, clashes roil Bangladesh as opposition vows poll boycott
“Bangladesh authorities are carrying out mass arrests of political opposition in a clear attempt to quash the opposition and eliminate competition ahead of the general elections”, the rights group said.
New York-based Human Rights Watch, who called it a “violent autocratic crackdown”, said at least 16 people have been killed since protests escalated in October, including two police officers.
More than 5,500 people have been injured.
There was no immediate response by the government to the rights group’s report, but authorities say that those arrested should face justice for a range of violent crimes.
Human Rights Watch based its report on multiple witness interviews, as well as analysis of videos and police reports.
It said it had found “evidence that security forces are responsible for using excessive force, mass arbitrary arrests, enforced disappearances, torture, and extrajudicial killings”.
A free election is impossible when the government stifles free expression and systematically incapacitates the opposition
“Diplomatic partners should make clear that the crackdown will jeopardise economic cooperation”, the rights group said.
Julia Bleckner, senior Asia researcher at Human Rights Watch, said credible elections could not be held under such conditions.
“A free election is impossible when the government stifles free expression and systematically incapacitates the opposition, critics, and activists through arbitrary arrests, enforced disappearance, harassment, and intimidation,” Bleckner said.
Bangladesh arrests more opposition leaders as PM rejects dialogue
Bangladesh arrests more opposition leaders as PM rejects dialogue
The BNP on Sunday said at least 16,625 members had been arrested since October.
They include most of its leadership, most notably the BNP’s de facto chief Mirza Fakhrul Islam Alamgir.
Prosecutors and lawyers said in the past two weeks at least 526 BNP officials and activists were convicted and sentenced, mostly in absentia, over what the party said were “trumped up” charges.