Security officials in Karachi say that at least six people have been killed and 16 others injured as police continue an operation against criminal elements in the Lyari area of the Pakistani port city.
The chief of a police station was killed and three other police officials injured when unidentified gunmen hurled hand grenades and opened fire on them, police said.
Chaudhry Aslam Khan, the senior superintendent of police who is supervising the operation, said that his colleagues had been attacked by people wielding hand grenades and a rocket launcher.
The police had made several arrests, he said.
Footage broadcast by local media showed police officers firing at unseen attackers.
Local media reported that several civilians were among those who had been killed in the violence overnight on Saturday into Sunday.
The Dawn newspaper identified the police station chief who was killed as being Fawwad Khan, and reported that five civilians had been shot dead near Cheel Chowk and Gabol Park, both in Lyari.
Law enforcement agencies began an operation against criminal elements in the area on Friday, but have met with strong resistance.
Six people were killed when the operation began and shops and other businesses in the area have been forced to close.
The operation is being carried out by the Karachi police's Crime Investigation Department in conjunction with the Frontier Constabulary and an elite unit of the police.
Lyari is a longtime political stronghold of the ruling Pakistan People's Party, which has consistently won national and provincial assembly seats from the constituency.
The latest violence, however, has caused some local leaders of the political party, and its associate the People's Amn [Peace] Committee, to threaten to defect from the party.
A city of more than 18 million, Karachi has a long history of violence, with political, ethnic, sectarian and criminal disputes often causing the city to erupt into a maelstrom of battles between locally influential gangs in various neighbourhoods.
According to the Human Rights Commission of Pakistan (HCRP), more than 300 people have been killed this year alone in politically-related violence.
Last year, the HRCP reported that more than 1,400 people in total had been killed in the city.