The violence erupted on Friday in Rawalpindi, the neighbouring city of the capital Islamabad, when a procession by Shiite Muslims to mark the most important day of the mourning month of Muharram coincided with a sermon at a nearby Sunni mosque.
"A curfew has been imposed in Rawalpindi city to avert further violence following the incidents on Friday," Waseem Ahmed, an official at the main police control room, told AFP.
"The curfew will remain until midnight on Saturday. The whole city has been closed down," he said.
Angry Shiite protesters attacked the Sunni mosque and seminary, torching its building and an adjacent cloth market.
Rival groups then attacked each other, TV cameramen and security forces and also fired gunshots.
The authorities deployed large numbers of troops in the city and later imposed a full curfew as soldiers patrolled the streets to stop protesters coming in from other cities.
All entry points into Rawalpindi were blocked, resulting in traffic chaos on Saturday morning that choked parts of the highways leading to Islamabad.
Hospital officials said that those wounded in the violence on Friday had multiple injuries.
"So far we can confirm the death of eight people from the violence. We received a total of 44 injured people and 13 of them had gunshot wounds," Qasim Khan, a doctor at Rawalpindi's district hospital, told AFP.
An official from the rescue department said that the rescue operation was still ongoing and they were yet to fully control the fire at the mosque and seminary.
"The firefighting still goes on. Mobile phone communication has been jammed, so we are unable to quickly update about the relief activities," Nazim Javed, a rescue worker, told AFP.
Pakistan is rife with sectarian clashes, with Sunni militant groups linked to Al-Qaeda and the Taliban often attacking gatherings by Shiites, who constitute some 20 per cent of the country's overwhelmingly Muslim population.
Pakistan had deployed heavy security all across the country for 10th of Muharram on Friday -- which is the death anniversary of Hussein, the grandson of the Prophet Mohammed -- to avert any terrorist attack on the mourning processions of Shiites.
The authorities had jammed mobile phone services as part of the security measures on the day, which hampered communication following the clashes. Cell phones in Rawalpindi are expected to remain unconnected for the weekend.
The Islamic month of Muharram lasts until December 3 and is a particularly fraught time.
Hussein was killed by the armies of the caliph Yazid in 680 AD and his death in Karbala in Iraq is mourned across the world by Shiites every year.
