Youths rioted in this southern city for a second straight day Wednesday to protest a homicide bombing that killed 56 people, which a top Pakistani official said was aimed at "eliminating" the leadership of a moderate Sunni Muslim group.

Police confirmed that a lone unidentified homicide bomber detonated an 11-pound bomb near Sunni dignitaries seated in a Karachi park Tuesday at a religious service with 10,000 other worshippers.

The service, to mark the birthday of Islam's Prophet Muhammad, was organized by moderate Sunni groups including the Tehrik group, whose top two leaders and a third senior official were among the dead.

Amid soaring sectarian tensions, hundreds of security forces blocked main roads and shut schools throughout Karachi, Pakistan's largest city, to prevent a repeat of Tuesday's riots that broke out after the bombing.

But a group of youths, apparently supporters of the Tehrik group, rampaged through a neighborhood, setting fire to a bus and two cars, and smashing shop windows before police aided by Islamic clerics brought the situation under control, said area police chief Shah Nawaz Khan.

Funerals for many of the victims were held throughout Karachi and attended by up to 5,000 people. Some chanted "God is great, and our leaders have attained martyrdom."

There has been no claim of responsibility for the attack, one of the deadliest in Pakistan, a close U.S. ally in its war on terror.

"We cannot pinpoint anyone at this stage," said Karachi Police Chief Niaz Siddiqui. "The investigation is following different avenues."

But Information Minister Sheikh Rashid Ahmed said the bombing was aimed at "eliminating the leadership" of the influential Tehrik group.

Numerous Tekrik leaders, including its founder, have been killed since forming more than seven years ago. The group promotes a moderate form of Islam and members are known to have close ties with Shiite Muslim groups.

Hard-line Sunni groups are opposed to more liberal organizations, like Tehrik, and their more moderate schools of thought. Simmering tensions between hard-line Sunni and Shiite groups have also been behind previous attacks.

At a tense news conference, senior Tehrik leader Mohammed Shahid Ghauri gave Pakistani authorities 48 hours to catch the culprits, "otherwise we are able to call on Sunni Muslims to respond in various ways." He did not elaborate.

Ghauri also called for an investigation to be headed by Pakistan's military intelligence services and demanded the provincial Sindh government be dismissed for "failing to keep the peace."

"The government did not provide enough security at the park and only a token police presence was there," said Shah Turabul Haq, who heads another moderate Sunni group, Jamaat-e-Ahle Sunnat. "This is sheer terrorism aimed at weakening moderate Sunni Muslims."

President Gen. Pervez Musharraf has condemned the attack and said the culprits "will not go unpunished."

Shopkeepers in Multan, a Punjab provincial city 230 miles north of Karachi, closed their doors to protest the bombing, while about 150 Islamic students staged a noisy rally at a busy intersection in the capital, Islamabad.

"We demand answers for the blood of these martyrs," the students chanted.

Past attacks have been linked to simmering Shiite-Sunni Muslim tensions or rows between moderate and hard-line Sunni groups, which regard public ceremonies marking the prophet's birth as offensive. Most attacks have been blamed on outlawed extremist groups, but claims of responsibility are rarely made.

Karachi has been the scene of several bombings and other attacks since Pakistan became a key U.S. ally after the Sept. 11, 2001, attacks in the United States.

In 1987, two car bombings in downtown Karachi, minutes apart from each other, killed 74 people.

On March 19, 2005, a bomb killed 43 people at a Shiite shrine in the southwestern Baluchistan provincial town of Naseerabad.