Troops have been called in to maintain order in the Pakistani city of Multan after a car bomb killed at least 40 people at a meeting of Sunni Muslims.
It is not yet clear who carried out the attack
The attack took place at the end of an all-night vigil to mark the first anniversary of the killing of militant Sunni leader Azam Tariq.
Police said a car exploded as thousands of mourners were leaving the gathering.
Last week, 31 people died in a bomb attack on a mosque used by Pakistan's minority Shia community.
Sectarian attacks on the Shias have claimed more than 100 lives in the past few months.
Tensions have been running high on the streets of Multan after Thursday's attack, which also injured more than 100 people.
Supporters of Sunni extremist groups are calling for revenge against the Shias they blame for the deaths.
Pakistani Interior Minister Aftab Khan Sherpao told the BBC the army had been called in as a precautionary measure to prevent further violence.
Soldiers are patrolling Multan's streets and have been deployed outside the city's hospital, where several hundred supporters of the outlawed Sunni group Millat-e-Islami had gathered.
"It seems to be an act of sectarian terrorism, but we are still investigating," Multan's deputy police chief, Arshad Hameed, told the Associated Press news agency.
The overnight meeting had been called to galvanise support for Millat-e-Islami and to commemorate the death of Azam Tariq, who was killed in an attack on his car outside Islamabad on 6 October last year.
The car bomb went off at about 0440 (2340 GMT Wednesday) as a large number of people started to walk towards the car park.
Several people died and dozens were injured on the spot, but more were injured in a resulting stampede.
"The explosion numbed our ears, we saw people falling on each other, everybody was crying, everybody was running," eyewitness Jamil Usmani said.
Witnesses reported people being torn to pieces and screaming for help. Large patches of blood stained the ground.
Doctors at the city's main hospital said there were more injured in the casualty ward than they could handle. Some of the injured died on the operating table and several people remain in a critical condition.
Pakistani Information Minister Sheikh Rashid condemned the attack.
"It is an act of brutal terrorism aimed at creating instability in the country," he told AFP.
Millat-e-Islami, formerly known as Sipah-e-Sahaba, 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 150 million people. Most of the rest are Shias.