Chief Justice Iftikhar Muhammad Chaudhry, suspended from his job by President Pervez Musharraf, had flown in to address a rally, but the violence meant he was unable to leave the airport.

Opposition groups blamed the MQM party, which runs Karachi, of organising the unrest, but it denies this. In the worst violence, supporters of the pro-Musharraf Muttahida Quami Movement (MQM) and activists from the Pakistan People's Party (PPP) of former Prime Minister Benazir Bhutto fought gun battles for an hour.

The private Aaj TV channel showed pictures of its office under fire. "We are under attack, " said journalist Talat Hussain on air, sheltering behind a wall. "We have seen no security force. No one has come to help us."

"It is state-sponsored terrorism.

The Sindh government is responsible but we are not going to back off, " said Sherry Rehman of the PPP. An MQM spokesman denied his party was involved in the violence at the TV station.

Since his suspension on charges of "misuse of authority", Chaudhry has become the focus of opposition to the government of President Musharraf, who took power in a coup in 1999. Chaudhry's supporters say that Musharraf wants the judiciary headed by a lawyer whom he can more easily manipulate.

He flew from Islamabad to Karachi on Saturday morning, planning to address a rally in the city but was unable to leave the airport, because roads were blocked. One plan was for him to travel to the city centre by helicopter, but Chaudhry insisted that he should be able to travel by road.

Musharraf has ruled out a state of emergency, and appealed to the country to stand united and peaceful.

The president himself was due to address a rally in Islamabad late last night.