Rival political workers fought intense gun battles ahead of huge rallies by Pakistan's suspended top judge and President Pervez Musharraf, leaving 34 people dead and scores more injured.
Black smoke billowed over the volatile southern city of Karachi as two months of tension over Musharraf's removal of defiant Iftikhar Muhammad Chaudhry boiled over into violence.
Mobs also torched a police post, four buses and dozens of other vehicles despite the presence of 15,000 paramilitary troops and police on the streets of the port city of 12m people.
The government's suspension of Chief Justice Chaudhry on 9 March has outraged the judiciary and the opposition and has blown up into the most serious challenge to President Musharraf's authority since he seized power in 1999.
Opposition leaders said the city was under siege by supporters of the pro-government Muttahida Qaumi Movement (MQM), the party that runs Pakistan's biggest city.
A senior Karachi policeman said: 'It is an extremely serious situation.'
A security official added that opposition parties supporting the chief justice and pro-Musharraf activists were exchanging gunfire in seven locations, one of them less than 1km from the city's airport.
