An apparent suicide bombing at a Pakistani mosque has left at least 12 worshippers dead and about 100 injured.

The attack came as hundreds of worshippers crowded into a popular Shi'ite mosque for Friday prayers in Pakistan's largest city, Karachi.

Initial police reports said that the bomber mingled with the crowd and blew himself up once he had entered the prayer hall, leaving a gruesome scene of destruction in his wake.

Hospitals across Karachi are reporting blood shortages, as they struggle to deal with the incoming wounded.

Afterwards, violent protests erupted throughout the city, with demonstrators accusing the government of not doing enough to stop terrorist violence.

Provincial Interior Minister Aftab Sheikh said that the attack happened while police guarded the mosque, indicating that better security is necessary, including bomb detectors. ?Places of worship should be provided more security and persons entering with bombs should be able to be detected,? he said.

Mr. Sheikh said that an investigation is under way and it is too early to determine a motive for the bombing.

While relations between Pakistan's majority Sunni and minority Shi'ite populations are normally good, militants from both sides stage occasional sectarian terror attacks.

In March, unknown assailants opened fire on a Shi'ite religious procession in the southwestern city of Quetta, killing more than 50 people.

Pakistani political commentator Ayaz Amir said that many of the country's recent terror attacks may be the result of violence spilling over from neighboring Afghanistan. ?Pakistan really is in the center of so much trouble that is being caused, not really within Pakistan, but across the border,? he explained.

Afghanistan faces an armed anti-government insurgency and Afghan militants are believed to have set up camps inside Pakistani territory.