Troops of Pakistan's paramilitary force rush to a bus damaged by bomb explosion in Karachi, Pakistan, on Friday. (Fareed Khan/Associated Press)

Two bombs targeting Shia Muslims exploded in Pakistan's largest city Friday, one outside a hospital treating victims from the first blast hours earlier.

At least 22 people were killed and more than 50 others wounded.

Police Chief Waseem Ahmad appealed to Shias in the teeming, chaotic city of more than 16 million to remain calm.

The most recent bomb was outside the entrance to the emergency ward at Jinnah Hospital, said witness Imran Ahmad. The first bomb was attached to a motorcycle, detonating as a bus carrying Shia Muslim worshippers drove by.

Local TV footage showed security officials examining the twisted metal pieces of the wrecked motorbike. The bus nearby appeared severely damaged. Windows in nearby buildings also shattered.

The bus carried mainly women and children headed to a Shia religious gathering, senior police official Javed Akbar said.

Dr. Simi Jamali said at least 11 bodies were brought to a local hospital.

Karachi has seen a number of attacks aimed at minority Shias in recent weeks, including one that killed dozens at a procession for a religious holiday and sparked subsequent riots. Extremists from Pakistan's Sunni Muslim majority have been blamed.

Pakistan's Sunnis and Shias generally live in peace, but extremists from the two sects have targeted one another's leaders and worshippers.

Karachi is a port city and the main commercial hub in Pakistan. It has a history of political, ethnic and religious violence. The Taliban, a Sunni Islamist movement, also are believed to use Karachi as a place to rest and raise money.