Angry Sunni Muslims have protested in the Pakistani city of Multan in defiance of a ban on public religious meetings imposed after a deadly car bombing.
About 40 people died when a militant Sunni meeting was attacked on Thursday.
Hundreds of protesters in Multan burned tyres, blocked roads and shouted slogans against Shia Muslims.
The chief minister of Punjab province has offered a 10m rupee ($175,000) reward for information leading to the capture of the attackers.
Pakistani police said they had arrested about a dozen Shia activists in Multan and Faisalabad and units were conducting further raids.
Pakistan has a long history of sectarian violence, particularly between majority Sunni and minority Shia Muslims.
Thursday's bombing came after a number of deadly attacks on Shias over the past few months.
Police and the army were put on high alert nationwide for Friday prayers.
The BBC's Shahid Malik in Multan says hundreds of Sunni Muslims protested in the Rashidabad area of the city.
They set tyres on fire and disrupted traffic but were later persuaded to disperse.
A protester at the rally, Mohammed Qasim, told Associated Press: "We are peaceful, but we will not remain so if the killers of our people are not arrested within a week. We know [Shias] killed our people."
Schools and businesses remained closed in Multan for a second day.
Schools in the eastern district of Sialkot, where 30 Shias were killed in a mosque bombing on 1 October, have also been closed for the day.
A three-day conference being organised by a religious body has been cancelled.
Banned
After Thursday's attack in Multan, Interior Minister Aftab Khan Sherpao issued a ban on public religious meetings except for gatherings for prayers inside mosques.
Mr Sherpao said the government had also ordered provincial administrations to ensure strict implementation of the ban already imposed on a number of extremist religious groups.
He said: "The government has directed the entire security apparatus to remain on a high state of alert because of the threat posed by elements trying to destabilise the country through acts of terrorism."
Thursday's explosion occurred after an all-night vigil organised by the militant Millat-e-Islami group, formerly known as Sipah-e-Sahaba, to mark the first anniversary of the killing of militant Sunni leader Azam Tariq.
Witnesses reported people being torn to pieces and screaming for help. Large patches of blood stained the ground.
Punjab Chief Minister Chaudhry Pervez Ilahi said he would be asking local authorities in Multan to explain why permission had been given for a banned organisation to meet.
Millat-e-Islami was banned by the government last year along with a number of other Sunni and Shia groups because of its alleged involvement in sectarian violence.
Sunni Muslims make up about 80% of Pakistan's 150m people. Most of the rest are Shias.