Gunmen killed at least 45 people in Pakistan's largest city after the assassination of a local politician set off a cycle of revenge attacks.
Dozens of vehicles and shops were set on fire as security forces struggled to regain control of Karachi.
Schools were closed and most business ground to a halt in the southern city of more than 16 million, Pakistan's main commercial hub.
Officials did not say who was to blame for the violence.
Karachi has a history of political, ethnic and religious violence and has long been a hideout for al Qaida and the Taliban.
The latest violence came after Raza Haider, a provincial politician, was shot dead along with his bodyguard in a mosque while preparing to offer prayers on Monday in the Nazimabad area.
Mr Haider was a member of the Muttahida Qaumi Movement (MQM), the political party that runs the city and represents mainly descendants of Urdu-speaking migrants from India who settled in Pakistan when it was created in 1947.
The MQM's main rival is the Awami National Party, a secular nationalist party whose main power centre is Pakistan's north-west and whose base is the ethnic Pashtun community living in Karachi.
Within hours of Mr Haider's assassination, gangs torched buildings in Karachi and gunfire erupted in several parts of the city. Many of the dead were killed in targeted attacks, authorities said. An investigation had been ordered into who was behind them.
Independent analysts say followers of all political parties in Karachi are heavily involved in criminal activities such as protection rackets and illegal land dealings. In some districts armed men linked to political parties stand guard at checkpoints. While violence is still common, the city was far more dangerous in the 1980s and 1990s when there was regular outbreaks of political and ethnic murders.