Pakistan has deployed army troops in Karachi, as more riots broke out ahead of Thursday's mass funerals for three leaders of a Sunni Muslim group killed in Tuesday's suicide attack.
Angry youth torched buses and cars and burnt tires in the streets, forcing shops in the city to close. Youths have been rioting in Karachi since the bombing that killed 57 people and wounded about 100 others.
Pakistani Prime Minister Shaukat Aziz visited Karachi and met with Sunni leaders in a bid to calm the situation. He vowed to bring the culprits to justice.
Two bombers detonated explosives Tuesday as thousands of followers were taking part in a mass prayer celebrating the Prophet Muhammad's birthday. There has been no claim of responsibility.
Pakistani President Pervez Musharraf has ordered increased security at religious sites.
Pakistani Interior Minister Aftab Ahmed Sherpao tells VOA there will be an exhaustive investigation into what he calls a gruesome crime.
Karachi has often been the scene of sectarian violence between Pakistan's dominant Sunni and minority Shi'ite Muslims.