At least 100 people have been injured after Bangladeshi police and opposition supporters clashed in the capital, Dhaka, witnesses say.
Police were attacked with rocks
Police used batons and tear gas to beat back thousands of protesters trying to blockade the city. Crowds threw stones.
One senior opposition figure, ex-army chief Gen KM Shafiullah, was critically injured in the street battles.
The opposition wants electoral reform and the resignation of the government of Prime Minister Khaleda Zia.
Sunday's action, banned by the High Court last Thursday, was part of a campaign against the government, which the main opposition Awami League and its allies say is incompetent and corrupt.
The Bangladesh National Party administration's term expires this October and general elections are due in January 2007.
Among those injured on Sunday were a number of police, thousands of whom had been deployed. Dozens of arrests were also made, the opposition said.
The Awami League has called for a 36-hour general strike beginning on Tuesday to protest against alleged police brutality.
The opposition says the "Dhaka siege" is aimed at cutting the capital off from the rest of Bangladesh to put pressure on the government.
General Shafiullah was badly hurt
Police used batons and tear gas to disperse thousands of opposition supporters who had gathered at about 20 points in and around the capital.
The fiercest clash took place in the suburb of Kanchpur, on a key highway that links Dhaka with the port city of Chittagong.
Witnesses say police first tried to beak up the protests there with batons and tear gas, but later fired rubber bullets to disperse the crowds. A number of vehicles were also set ablaze.
Gen KM Shafiullah, who is a senior leader of the opposition Awami League, was injured in the violence. He has been admitted to a military hospital in Dhaka where his condition is said to be critical.
Opposition sources say police attacked him with batons.
The clashes at Kanchpur also left another senior opposition figure, Mohiduddin Khan Alamgir, injured. Mr Alamgir, a former planning minister, was also taken to hospital.
"Police attacked peaceful demonstrators," opposition MP Abdus Shahid, who suffered a leg injury, told the Associated Press (AP) news agency.
About 20,000 security personnel had been deployed to prevent violence, Bangladesh police chief Abdul Qaiyum said on Saturday night.
"We will do whatever required to keep the law and order under control," he said, AP reported.
Opposition groups want the government to agree electoral reform proposals to ensure the next general election is free and fair.
They are also demanding the resignation of chief election commissioner MA Aziz, who they accuse of not being neutral. Mr Aziz denies the allegations.
"The siege is our democratic right and we are going to implement it," Awami general secretary Abdul Jalil said, Reuters news agency reported.
Prime Minister Zia has promised to stay in office until October.
Under the constitution, she must hand power to a caretaker administration to oversee the next general election.
Her government has blamed the opposition for deadlock in talks on electoral reform.