At least 25 people have been killed and dozens injured in a suspected suicide bombing at a mosque in the eastern Pakistani city of Sialkot, police say.
Troops were called in to keep order
Hundreds of worshippers of the Muslim Shia minority were packed into the mosque attending Friday prayers.
There have been angry protests in Sialkot and Karachi. A second bomb at the scene did not explode, police said.
About 100 Shias have been killed in sectarian violence in Pakistan this year alone.
The BBC's Paul Anderson in Islamabad says suspicion for the Sialkot attack will fall on one of several extremist groups from the Sunni majority.
President Pervez Musharraf has vowed to root out those behind the attack, saying it showed "that terrorists have no religion and are enemies of mankind".
Police said the blast at the Mistri Abdullah Imambargah mosque was caused by a device concealed inside a briefcase.
The blast took place in the centre of the prayer hall, causing chaos.
"I was praying when I first saw a bright light and then something exploded with a big bang, and I fell down," said Sajjad Anwar, 36, who was being treated at a hospital.
"My mind stopped working for a while after the blast, but when I
opened my eyes, I was lying among dead bodies," said Mumtaz Ali Shah, 43.
Police said the bomb left a two-foot deep crater with the dead and wounded strewn across the floor.
A second bomb weighing about 5kg was found in a briefcase but was defused by a bomb disposal squad.
Hundreds of angry Shias went on the rampage after the explosion, throwing stones at officers and attacking property.
PAKISTAN'S SECTARIAN DIVIDE
Shias revere Ali, son-in-law of the Prophet Mohammed
Pakistan is 20% Shia, 70% Sunni
Violence between Sunni and Shia factions began in early 1980s
More than 150 people have died in the past year alone
About 4,000 people have been killed in total
Most violence takes place in Sindh, Balochistan and Punjab
They torched police vehicles, set fire to part of a hospital and blocked roads and the railway line. Troops were deployed onto the streets to restore order.
Reports say police tried to push back the crowd to allow the wounded to be taken to hospital.
The blast comes just days after security was stepped up in major Pakistani cities amid fears of reprisals for the killing by Pakistani security forces of leading al-Qaeda suspect Amjad Farooqi.
Pakistan Information Minister Sheikh Rashid Ahmed said: "It could be a possible reaction to that killing. It is an act of terrorism aimed at destabilising the country."
Correspondents says Sialkot is not a known centre for sectarian violence.
At least 20 people were killed in a suicide bombing on a Shia mosque in Karachi in May.
That attack followed the killing of a leading Sunni cleric.