Some unidentified gunmen in Pakistan opened fire near the Chinese consulate in Karachi, killing two policemen, media reports said.
An explosion, the nature of which was not immediately known, was also heard in Clifton area, Dawn news reported.
Two policemen have been killed in an exchange of fire, a senior police officer said.
The operation was still underway, the report said.
Heavy contingents of police and paramilitary forces have been deployed in the area after the exchange of fire.
The area, considered a red zone, is heavily guarded.