Pakistan on Friday ordered 1,000 extra troops to deploy in Karachi with instructions to shoot-to-kill after another 65 people were killed in the deadliest six months of political violence since 1995.
Gunfire reverberated in several neighbourhoods and thousands of people were stranded, short of food and too frightened to go out after three consecutive nights of violence in what is Pakistan's biggest city and economic hub.
The US ambassador to Pakistan voiced concern about increased instability in the city whose Arabian Sea port is used by the United States to ship supplies to the 150,000 foreign troops fighting the Taliban in Afghanistan.
The killings have been blamed on loyalists of former coalition partners the Muttahida Qaumi Movement (MQM) and Awami National Party (ANP), which represent different ethnic communities and straddle volatile political fault-lines.
Karachi essentially shut down on Friday with shops closed and the dominant local party, MQM, calling for a day of mourning, and protest rallies.
"At least 65 people have been killed in the violence since Tuesday. The number of injured is around 100," Sharjeel Memon, the information minister in the southern province of Sindh where Karachi is the capital, told AFP.
A security official confirmed the toll.
In the worst incident, gunmen opened fire on two buses, killing 12 people, including a six-year-old girl overnight, a security official said.
Memon said the government had ordered security forces to "shoot on sight" armed men involved in the attacks.
Interior minister Rehman Malik said the government would deploy an extra 1,000 troops on the streets.
"We are bringing 1,000 more paramilitary troops to control the situation in Karachi," Malik told reporters, after Pakistan's leading human rights commission criticised government inaction over the violence.
Malik called for "targeted action" against the killers, but said there would be no large-scale operation in the city of 16 million.
"We know which forces are behind these killings. We have satellite records of the areas where terrorists are killing innocent people," said Malik, comparing those responsible to Taliban insurgents in the northwest.
"These militants are no lesser evil than the Taliban. They are killing people to destablise the democratic system".
The Human Rights Commission of Pakistan says 490 people have been killed in targeted killings so far this year, compared to 748 in 2010 and 272 in 2009. This year is considered the worst bout of violence in Karachi since 1995.
In 1995, 1,742 people died in violence blamed on ethnic, sectarian and political tensions, says the state-funded Citizen-Police Liaison Committee.
The US ambassador to Pakistan, Cameron Munter, issued a statement saying that the embassy was "deeply concerned about the escalating violence".
"We call on all parties to refrain from further violence and work toward a peaceful resolution of differences," he said.