Dozens of gang members angered by the killing of one of their leaders opened fire on passers-by outside a market in southern Pakistan, killing 15 people.
Machine guns and grenades were used in the attack on Wednesday in the Lyari neighborhood of Karachi, in which more than 40 other people were wounded, senior police officer Shah Nawaz said.
Nawaz said a notorious gang was behind the attack, and a major search was under way to find and arrest all those who had killed "innocent women, children and men".
"They will not go unpunished," he said.
Another police officer, Abdul Khaliq Sheikh, said police and paramilitary rangers chased the gunmen and killed eight in a gun battle. He said supporters of a gang leader killed overnight in a shoot-out with police were behind the violence.
Television footage from the scene showed ambulances taking the wounded to hospitals in Pakistan's largest city.
Lyari is considered one of Karachi's more dangerous neighbourhoods, where rival gangs often battle each other.
A January report by the International Crisis Group said gang members in Lyari operate like warlords and inter-gang warfare has included the use of mortars and other heavy weapons.
Hundreds of people have been killed in Lyari since 2009, and periodic operations by the paramilitaries and police result in days of armed clashes, disrupting civilian life but not ending the violence, the report said.