A bomb explosion has killed at least 15 people and injured nearly 40 at a Shia mosque in the southern Pakistani city of Karachi.
Dozens of worshippers were injured in the blast
President Pervez Musharraf responded by pledging to take firm action to deal with inter-Muslim sectarian violence.
The blast happened during evening prayers, causing extensive damage to the building.
On Sunday a top Sunni Muslim cleric, Mufti Nizamuddin Shamzai, was shot dead as he drove to his religious school.
Thousands of Pakistani troops had been deployed on the city's streets after the killing sparked rioting on Sunday.
Sunni-Shia sectarian violence has killed as many as 4,000 people in the past 15 years.
Mr Musharraf said he would take tough measures to restore order in the city, but did not specify what they would be.
"I will take serious action, this is the second incident
within 24 hours, following Mufti Shamzai's killing," Interior Minister Sheikh Rashid Ahmed quoted him as saying.
The explosion was so powerful it knocked down a pillar and one of the side walls of the mosque.
An office building belonging to the trust that manages the mosque collapsed.
An emergency was declared at three hospitals and appeals were made for blood donations.
Police had being trying to protect Shia mosques
Hundreds of angry Shia youths rioted after the bombing, setting fire to two police vehicles and a petrol station, and ransacking offices.
Police fired tear gas in response.
The blast took place only 1km from the site of Mufti Shamzai's killing.
Police had been trying specifically to protect Shia mosques following Mufti Shamzai's death.
Interior ministry spokesman Abdur Rauf Chaudhry said it was too early to say who carried out the attack but said it appeared to be sectarian violence.
The authorities were investigating reports that a person pretending to be a worshipper had left a briefcase in the mosque shortly before the explosion.
A coalition of six fundamentalist parties, the Muttahida Majlis-e-Amal (MMA), had called for a nationwide strike on Friday in protest at Mufti Shamzai's killing.
A senior MMA leader, Maulana Fazlur Rahman, said: "We will not sit silent. We will protest against the brutal murder of a great religious personality.
The car of Mufti Shamzai was ambushed by up to six gunmen
"We will be forced to take other steps if the killers of Nizamuddin Shamzai are not arrested," he said.
A police taskforce has been formed to investigate the killing.
No one has yet said they carried out the attack.
Up to six gunmen ambushed the car of Mufti Shamzai, witnesses said.
One of the 75-year-old cleric's sons, a nephew, a driver and a police bodyguard were wounded.
Sunni Muslims, including his students, rioted at news of his death, attacking a police station and burning cars.
The cleric's death came three weeks after at least 14 people were killed in Karachi when a man, apparently dressed as a Shia cleric, blew himself up in a Shia mosque.