Violence erupted in different parts of the port city after an unidentified man opened fire in a Mohajir neighbourhood in the centre of the city.
Officials at Karachi's largest hospital said it had received 25 bodies while a senior police official said nine bodies were delivered to another hospital.
Dozens of cars and several shops were burnt in the riots.
Al Jazeera's Sohail Rahman, reporting from Islamabad, said two supporters of the Muttahida Qaumi Movement, a party generally representing the Mohajirs, had been found shot in the northern suburbs.
"There has been indiscriminate firing in the northern suburbs of the city with six police officers wounded," he said.
"It is very unsure how the police will be able to respond to the violence."
Karachi, Pakistan's financial capital, has a long history of ethnic, religious and sectarian violence but has been relatively peaceful in recent years.
The city is dominated by Mohajirs, Urdu-speaking people who migrated from India after Pakistan was created in 1947, but there is also a sizeable population of ethnic Pashtuns.