Bangladesh police fired tear gas and used batons to stop thousands of Opposition activists trying to march to the prime minister's office on Tuesday to demand electoral reforms, witnesses said.
At least 100 people were injured in the clashes that erupted in Dhaka after opposition party members tried to break past barbed-wire barricades around Begum Khaleda Zia's office.
Protesters also exploded crude bombs at several places in the city, but there were no reports of any casualties or damage, witnesses said. Several police vehicles were attacked with stones.
"The violence has been widespread, with protesters fighting police and attacking vehicles," one witness said.
The government did not make any immediate comment on the violence in the city.
The Opposition had planned to lay a siege around the prime minister's office to force her to accept reforms for a free and fair national election in January 2007.
The Opposition wants the chief election commissioner and his deputies removed, accusing them of a pro-government bias.
The parties also want a say in choosing the head of a caretaker administration that will supervise the election.
Khaleda has rejected the demands, saying the Opposition was trying to destroy democracy and push the country into anarchy.