A powerful car bomb ripped through a prayer gathering of Sunni Muslim radicals in central Pakistan, killing 41 people and wounding more than 100 as the country grappled with a new upsurge of sectarian violence.
The blast at 4:30 am (2330 GMT Wednesday) targeted around 2,000 Sunnis from an outlawed group who had gathered in the city of Multan, a hotbed of Islamic extremism, to mark the first anniversary of the assassination of their leader.
The pre-dawn attack came six days after 30 worshippers from the rival Shiite minority were killed by a suicide bomber as they prayed in the eastern city of Sialkot. Police had feared a revenge attack.
It also falls a week before the start of the Muslim holy month of Ramadan, raising fears that a renewed cycle of bloodshed between the sects could continue into the tense fasting month.
"It was a high intensity (car) bomb which carried seven to eight kilograms of explosives," city police chief Sikandar Hayat told AFP. It was set off by remote control or a timer.
Thursday's carnage brings to six the number of sectarian attacks this year and raises the death toll to 165, one of the highest in two decades of sectarian bloodletting.
The target was the most radical of Pakistan's hardline Sunni militant organisations, Millat-e-Islamia, the new name for the outlawed Sipah-e-Sahaba led by the late Azam Tariq. The group is accused of killing hundreds of Shiites since the 1980s.
The explosions triggered a stampede through Multan, an ancient shrine-dotted city on the southern Punjab plains surrounded by cotton and sugarcane fields.
The main Rasheedabad Square was littered with body parts, turbans and bloodstains, witnesses said.
Forty people were killed, Imran Rafiq, the chief medical officer of the city's main hospital told AFP. More than 100 were injured and extra hospital staff were brought in to extract shrapnel from the wounded.
Local police officer Mohammad Nazakat said another person died in the hospital later, bringing the death toll to 41.
No one has claimed responsibility but Sunni leaders immediately blamed Shiite extremists.
"This attack is carried out by Shiites," Millat-e-Islamia leader Muhammad Ahmed Ludhianvi told AFP.
Shiite leader Allama Syed Taqi Naqvi denied any hand in the blasts. "It is a terrorist act. No Shiite was involved. Those killed were our brothers because they were Muslims."
Police have detained several people for questioning, police chief Hayat told AFP.
The prayer gathering in Rasheedabad Square had lasted the whole night and the crowd was beginning to disperse when the pre-dawn calm was shattered by the explosion.
Multan is known as the "city of saints" for the numerous Sufi Muslim saints buried in glittering blue-tiled shrines, but has long been a hotbed of Islamic extremism.
Information Minister Sheikh Rashid condemned the attack as "an act of brutal terrorism".
Interior Minister Aftab Sherpao said authorities had deployed regular troops in the city to maintain order.
"The federal government has decided to advise the provincial governments to impose ban on processions, religious gatherings and congregation until further orders, except for gatherings for prayers inside mosques," he told reporters in Islamabad.
Outside the hospital victims' relatives chanted slogans against Shiites and vowed revenge. Hundreds of police and paramilitary troops were deployed to guard mosques and religious schools in several cities to prevent follow-up violence.
Millat-e-Islamia's Ludhianvi called a two-day mourning period for Thursday and Friday.
Hundreds of people attended funeral prayers for 33 of the victims, held in a ground next to the hospital. Prayers for seven others who were killed on the spot were held elsewhere in the city. The bodies were transported to their home towns for burial, police said.
The Sunni majority and Shiite minority for the most part live together peacefully. But militants from each sect have been involved in bitter tit-for-tat violence since the 1980s. The conflict has so far cost more than 4,000 lives.
Sunni extremist leader Tariq was shot dead in a hail of gunfire on October 6 last year as he drove into Islamabad.
Police blamed Sipah-e-Sahaba and another outlawed Sunni group Lashkar-e-Jhangvi for last Friday's attack in Sialkot.