A wave of deadly political and ethnic violence gripped Pakistan’s largest city for the fourth straight day Friday, and police and paramilitary troops were given orders to shoot suspected assailants on sight.

As many as 95 people have been killed in Karachi since Tuesday in assassinations, shooting rampages on buses and arson attacks, according to law enforcement authorities, who were widely faulted for doing little to stop the carnage. The fighting spread from one multi­ethnic, lower-middle-class district to other parts of the seaside metropolis, and by Friday the city was under near-lockdown as armed men fired from windows and rooftops.

Karachi, a commercial hub of 18 million people where various ethnic and political groups compete for land and votes, has for decades been the scene of gang-style bloodshed. But this week’s fighting ranks among the deadliest, and many of those killed were ordinary residents, including children and elderly people used as “human shields,” federal Interior Minister Rehman Malik said.

Late Friday, paramilitary troops in armored vehicles began evacuating people from the most violence-wracked neighborhoods, according to Pakistani media. Residents in those areas described anarchic scenes of men prowling empty streets, tossing grenades and launching rockets. Shots were fired at passersby, ambulances and television crews, whose presenters wore flak vests while reporting outside.

Authorities said at least 34 people were killed Thursday, becoming the latest victims in a year in which the Human Rights Commission of Pakistan says at least 1,138 have been slain in Karachi.

The latest spasm of violence began Tuesday, after the killing of an activist belonging to the Awami National Party, a political party that mostly represents Pashtuns originally from the Afghanistan-Pakistan border region.

Law enforcement authorities — who are often accused of being beholden to or cowed by the city’s political groups — declined to place blame, and Malik said only that he “knew” who was behind the violence, which he described as a plot to destabilize Pakistan. But bloodshed in Karachi is typically attributed to the Awami party’s rivalry with the dominant Muttahida Qaumi Movement, or MQM, which represents descendants of Urdu-speaking immigrants from India.

Other political parties, including the ruling Pakistan People’s Party, and sectarian organizations also vie for influence in the city. On Thursday, Malik said 1,000 paramilitary troops would be deployed to help curb the violence.

But residents said Friday that security officials’ response seemed tepid. Asghar Hussain, 36, who lives in Qasba Colony — where the fighting began — said his neighbors’ house had been torched and looted. “The police and [army] rangers have not taken any action against these criminals,” he said.

The U.S. ambassador to Pakistan, Cameron Munter, condemned the bloodshed in a statement Friday. “We call on all parties to refrain from further violence and work toward a peaceful resolution of differences,” Munter said.

Also Friday, Pakistan’s government rebuked as “extremely irresponsible” an assertion made a day earlier by the top U.S. military officer, Adm. Mike Mullen, that tied Islamabad to the recent killing of a Pakistani journalist. Mullen told reporters he believed the government had “sanctioned” the killing of Syed Saleem Shahzad, who had written about Islamist militants’ infiltration of Pakistan’s powerful security forces.

The New York Times reported this week that Obama administration officials have intelligence indicating that Pakistan’s top spy agency killed Shahzad to intimidate Pakistani reporters. That view is widely shared by journalists and human rights activists in Pakistan, who accuse intelligence officials of abducting, torturing and killing critics and opponents.

Special correspondent Nisar Mehdi in Karachi contributed to this report.