People were arrested at bus and train terminals Friday following reports that people from different districts were gathering in Dhaka, the BBC reported.
Police called the detentions routine.
But the opposition alleges the arrests have been carried out to foil its anti-government initiatives. It set an April 30 deadline for the government to resign and call early elections.
"It's not acceptable that anyone sets a deadline for the fall of an elected government," said Haris Chowdhury, the political secretary of Prime Minister Khaleda Zia.
He said that a rally outside Dhaka, where the opposition leader Sheikh Hasina spoke, proved that there was no bar on democratic practice in Bangladesh.