By Asif Farooqi, IOL Correspondent

ISLAMABAD, May 31 (IslamOnline.net) - A massive explosion in a crowded Shiite mosque in the southern port city of Karachi Monday, May 31, killed at least 12 and injured more than 30 people, some of them seriously, police and hospital sources said.

Initial reports said that a powerful bomb went off in Ali Reza Mosque in downtown Karachi during evening prayer when the mosque has most worshippers.

Doctor Azmat Ali Abdi of National Hospital Karachi told IslamOnline.net that he had received six dead bodies and 12 injured with serious injuries.

Another hospital close to the spot of the incident received another six bodies and many injured, Abdi said over phone.

The doctor said the death toll is most likely to increase because many of the injured were in serious condition.

Police officials were rushing to the place but they did not offer a comment on the causes of explosion.

This is the third attack on Shiite mosques in Pakistan during the last month in which dozens of Shiite Muslims have been killed.

On Sunday May 30, six people assassinated the most revered Pakistani scholar Mufti Nizamuddin Shamzai as he drove out of his residence in the Binouri Town section of Karachi.

A police guard was killed and at least 32 other people injured Wednesday, May 26, when two car bombs exploded near U.S. buildings in Karachi.

At least 13 worshippers were killed and around 100 others injured in a "suicide bomb attack" on a packed Shiite mosque in the city during prayers Friday, May 7.

No one has claimed responsibility but police blames the attacks on radical Sunni Muslims.

Both Shiite and Sunni sects of Muslims have been involved in sectarian violence in the country, especially in Karachi.

Shiites form 20 percent of the country's 145 million population. Sectarian violence involving militants from the two sects have claimed more than 4,000 people over the past several years across the country.