The violence is the worst so far from a political crisis shaking the military-dominated government of President Gen. Pervez Musharraf, a vital U.S. ally who provoked the turmoil by ousting the head of the Supreme Court.
Trouble broke out as Chief Justice Iftikhar Mohammed Chaudhry arrived for what organizers hoped would be the largest in two months of rallies by lawyers and opposition parties calling for his reinstatement and for Musharraf to step down.
Pro-government parties responded with their own show of strength, sparking street battles and rioting.
In the bloodiest episode, activists traded assault rifle fire about half a mile from the airport. Associated Press reporters saw the bodies of five men lying in the street, four near a shot-up car, the other next to the red flag of an opposition party.
Musharraf, speaking ahead of his own rally late Saturday in the capital, Islamabad, urged the nation to stand united and remain peaceful. He ruled out calling a state of emergency to contain the escalating unrest.
In Karachi, opposition activists accused a pro-government party, the Mutahida Qami Movement (MQM), of attacking them with batons and gunfire as they attempted to greet the judge at the airport.
An AP reporter saw MQM supporters calling for ammunition and firing from buildings, reportedly at supporters of the Pakistan's People's Party and Jamaat-e-Islami. Opposition supporters were firing back. An MQM leader, Farooq Sattar, said four of the party's supporters died in the clash.
Doctors at the city's four main hospitals said 27 people were dead and more than 100 injured, many of them from gunshot wounds. A senior security official in Karachi, speaking on condition of anonymity because he is not authorized to speak publicly, also said 27 had died.