Arrests have continued even after the ruling Awami League party and its allies won the largely uncontested elections held Jan 5, it said.
The US, the European Union and others refused to send observers to Bangladesh’s divisive polls.
Dozens and perhaps hundreds of opposition leaders and members were taken into custody before the elections, which were boycotted by the main opposition alliance led by the Bangladesh Nationalist Party (BNP).
BNP leader and two-time former Prime Minister Khaleda Zia was placed under de facto house arrest, with security forces surrounding her residence and blocking people from entering and leaving.
Many opposition leaders and activists have gone into hiding.
“While in some cases the government has acted appropriately to stop violence by some opposition forces, this spate of arrests is part of a pattern of weakening critics, limiting dissent, and consolidating ruling party power,” said Brad Adams, Asia director at Human Rights Watch.
“The Awami League claims that it is the leading democratic party in Bangladesh, but there is nothing democratic about this kind of widespread crackdown on critics.”
As many as 150 people have been killed in political violence in recent months, much of it carried out by opposition activists.
The Election Commission said that nearly 500 voting centres had to be closed because of the violence, including arson. Awami League activists have also engaged in attacks on opposition supporters.
Attacks on Hindu communities, allegedly by supporters of the BNP and the Jamaat-e-Islami, have been reported in the media.
Many Hindus, Bangladesh’s largest religious minority, traditionally vote for the governing Awami League Party.
At least 18 people died as a result of election-day violence, many of them having been shot by the police, according to media sources.