Abdul Jalil, general secretary of the Awami League, said police at ferry, bus and railway terminals were obstructing thousands of demonstrators heading for Dhaka to demand the resignation of the prime minister.
Opposition parties recently mounted a series of street protests and general strikes, accusing the government of fixing voting lists in its favour and failing to curb rising Islamic militancy.
More than 500 Awami League activists rallied outside its party headquarters in central Dhaka on Saturday to protest against the reported detentions and harassment of activists.
The government vowed to remain in power until its five-year term expires in October.
News media quoted unidentified officials as saying on Saturday that the government has instructed police to round up dissidents and restrict transport.
Authorities denied arresting opposition members and said they were detaining crime suspects, but not any political activists.
Police headquarters declined to comment.