At least 24 people were killed in overnight clashes between rival ethnic factions in Pakistan's southern port city of Karachi, media reported on Sunday.
The violence pitted the majority Urdu-speaking population, which migrated from India after creation of Pakistan in 1947, and Pashto-speaking people who have moved from north-western Khyber-Pakhtunkhwa to work in Karachi, dpa reported.
lashes began ahead of important by-elections on Sunday for a provincial seat of southern province of Sindh, of which Karachi is the capital.
Local broadcaster Geo reported that 24 people had died in sectarian clashes that started on Saturday evening. More than 50 people were injured, it said.
"The city is tense, shooting is going on in various areas and police have so far failed to control the killings," Geo said.
The seat fell vacant after Raza Haider, a lawmaker representing Urdu-speaking Muttahida Qaumi Movement, was killed last month. The party blamed the Pashtun-based Awami National Party (ANP).
That killing led to widespread violence which claimed around 100 lives.
The latest clashes appeared to be triggered by ANP's last-minute decision to pull out of the elections, to press for postponement of polls due to the poor security situation.
The government refused to delay Sunday's elections, and deployed hundreds of police and paramilitary troopers at 84 polling stations to thwart violence.
Karachi is Pakistan's financial hub, with a population of 18 million.