Parts of Pakistan's largest city shut down on Wednesday after an attack on a scrapyard pushed to 51 the number of people killed in four days - a spasm of violence illustrating the government's inability to stop the crime that has risen alongside Islamist militancy.
Some shops were set on fire in an outlying neighbourhood of Karachi, where police tried to calm gathering crowds, footage broadcast on Pakistani TV stations showed.
The violence in Karachi comes as Pakistan is engaged in talks with the US on the future of their shaky alliance against the Taliban and al-Qaida. US officials in Washington are expected to discuss on Wednesday a long-term military and security assistance pact with a visiting Pakistani delegation.
Karachi, a port city of some 16 million, has a long history of political, ethnic and religious strife, but this year has been exceptionally bloody. As of June, around 300 "targeted killings" had occurred in the city, roughly twice that of 2009. Many of the killings in Karachi have been linked to gangs allegedly controlled by political parties. The wave of violence in the city has coincided with Sunday's election to replace a provincial lawmaker killed in August.
Because of its status as the country's main economic hub, keeping Karachi calm is of prime importance to Pakistani leaders. A major chunk of supplies for US and NATO troops is shipped to the city before travelling overland in Pakistan and into neighbouring Afghanistan. And al Qaida and Taliban fighters are believed to frequent Karachi to rest and raise funds.
In the latest attack, gunmen opened fire in the scrapyard, which was in a commercial market area, killing 11 people late on Tuesday. The dead included eight Pakistanis of Baluch descent, said Sharmila Farooqi, a provincial government spokeswoman.
Ms Farooqi said police detained 55 suspects in connection with the latest violence, and that some were linked to local political parties. Security forces were patrolling the city to prevent fresh violence on Wednesday, she said. In many neighbourhoods, businesses shut down, while public transportation was scarce.
Footage on Pakistani TV showed several small shops, including fruit and vegetable stalls, ablaze in Malir, a neighbourhood on the outskirts of the city. Young men ran onto the streets, and police officers tried to keep the situation under control, the footage showed.
The two parties most linked to violence in Karachi - the Muttahida Quami Movement and the Awami National Party - have their electoral bases in different ethnic groups that make up a large share of the city's population.
The MQM claims to represent the Urdu-speaking descendants of those people who came to Karachi from India soon after the birth of Pakistan in 1947. It is secular and likes to speak out against the so-called Talibanisation of the city, a jab at the Awami National Party, which represents the ethnic Pashtuns from the Taliban heartland in the north-west.