One demonstrator was killed and 50 were injured when police fired tear gas to disperse opposition activists in the Bangladesh capital Dhaka on Thursday, police and witnesses said.
Demonstrators from the opposition Bangladesh Nationalist Party (BNP) set fire to a police vehicle during a march through the capital to denounce mass arrests of student activists.
Police fired tear gas and moved against the protesters with batons.
"The man was run over by a bus while trying to run for cover during police action," a BNP leader told reporters.
Demonstrators suffered injuries after being struck by batons or falling while trying to avoid the police charge.
The BNP official said police acted against protesters when they were already returning from a protest venue. Police said they acted to protect public property.
The protest was one of dozens called across the country in response to arrests prompted by campus unrest. Demonstrators said they were also unhappy at rulings barring BNP members from addressing parliament.
Other rallies were generally peaceful.
It was the first countrywide protest since Prime Minister Sheikh Hasina returned to power after a December 2008 parliamentary election that ended a two-year period of rule by an army-backed interim government.