Rival ethnic-based political factions have clashed in the Pakistani commercial hub of Karachi and at least 17 people have been killed, reviving worries of the return of the violence that plagued the city in the 1990s.
The city of about 18 million people is Pakistan's main industrial base and home to its main port, stock exchange and central bank. It is also the main gateway for Western military supplies bound for neighbouring land-locked Afghanistan.
Most foreign companies investing in Pakistan also have offices in the city.
Karachi has not been spared Islamist militant violence with several big bomb attacks over the past few years but for many residents, a bigger worry is the return of the factional violence that racked the city for much of the 1990s.
The cause of the latest round of violence was not clear but it involved activists of the Muttahida Qaumi Movement (MQM), the dominant party in the city, and the rival, ethnic Pashtun-based Awami National Party (ANP).
"Over the past 24 hours, at least 17 people have been killed in targeted attacks as well as in exchanges of fire between rival groups," Karachi police chief Waseem Ahmed told Reuters.
Stock market investors keep a wary eye on the tension and dealers said the latest bloodshed was one reason behind negative sentiment on Thursday with the benchmark Karachi Stock Exchange 100-share index <.KSE> down 0.50 percent by 0510 GMT.
The MQM draws most of its support from Urdu-speaking descendents of families that moved to Pakistan from India when the sub-continent was divided at the end of British rule in 1947.
Several million Pashtuns have migrated to the city over the years and many of them support the ANP.
Both parties are members of the ruling coalition federal government, led by President Asif Ali Zardari's party, but they compete for influence in Karachi.
Factional violence has flared occasionally over recent years and scores of people have been killed but it has not spun out of control.
Government officials say criminals, including drug lords competing for turf in the city's teeming neighbourhoods, take advantage of the tension, exacerbating the difficulties facing the police.