At least 42 people have been killed and over 100 wounded in an attack on Shia Muslims in the Pakistani city of Quetta, hospital officials say.
The explosion and gunfire created chaos
An explosion was followed by intensive gunfire as a Shia procession passed through the business district.
Doctors say the condition of many of the injured is critical. The city is under indefinite curfew.
The attacks came as Shia Muslims held ceremonies to mark the slaying of the Prophet Mohammed's grandson.
The exact number of dead remains unclear - some estimates put casualty figures higher, and doctors say they expect the death toll to rise.
By late on Tuesday the provincial government had confirmed 25 people killed in the violence. It said five of them were policemen, and that more than 100 others had been wounded.
Another shooting incident during a Shia procession in Punjab province left two people dead.
Another 40 were injured following clashes between Shias and Sunnis in Phalia, a small town more than 600 km (375 miles) north-east of Quetta.
No group has said it carried either attack, but sectarian violence has been a regular feature in Pakistan in recent years.
The BBC's Zaffar Abbas in Islamabad says it is unlikely the violence in Pakistan is linked to attacks on Shias in Iraq which left scores dead.
The Quetta explosion - on the climax of Ashura, the tenth day of the holy month of Muharram - created chaos.
It was followed by gunfire from different directions, witnesses say.
Eye-witnesses said as the procession was passing through the city's shopping district, a grenade was thrown, followed by firing from automatic guns.
"I was present near the procession when we first heard an explosion and then some people fired shots," Quetta Mayor Abdul Rahim Kakar said.
Afterwards groups of angry Shias attacked shops, vehicles and government property before security forces fired shots and tear gas to disperse the crowd.
Several cars parked in the area were also set on fire.
Reports say the atmosphere in Quetta remains tense.
Troops poured in to restore order
The mayor of the city said a curfew had been imposed and troops were patrolling the streets.
A senior Shia leader in Pakistan, Allama Hassan Turabi, has demanded that President Musharraf sack government officials including the interior minister for failing to prevent Tuesday's attack.
"This is not the first attack against us," he said, "our people are not safe in their homes. They are not safe in their mosques."
President Musharraf has condemned the Quetta attack and directed the authorities to take all action necessary to bring those guilty to justice.
Security has been tightened across Pakistan during the holy month of Muharram to prevent violence between the minority Shia and majority Sunni communities.
On Tuesday, there were big Shia processions in most towns and cities across the country to commemorate the seventh century martyrdom of the Prophet Mohammed's grandson, Imam Hussein.
SHIA FESTIVAL: ASHURA
Annual Shia festival commemorating martyrdom of Imam Hussein
Hussein, grandson of Prophet Mohammad, killed at Karbala by army of Caliph Yazid in 680
Faithful strike themselves with chains and swords to atone for Hussein martyrdom
The murder 19 years earlier of Ali, Hussein's father, gave rise to the central schism in Islam between Sunni and Shia
The authorities deployed thousands of security personnel to ward off attacks on the Shia day of mourning.
Last July, Quetta was the scene of one of the worst outbreaks of sectarian violence in Pakistan, when attackers armed with machine guns and grenades stormed a Shia mosque, killing 50 people who were praying inside.
Most of Pakistan's Sunni and Shia Muslims live peacefully together, but small radical groups on both sides are responsible for frequent attacks.
About 97% of Pakistan's population is Muslim, and Sunnis outnumber Shias by a ratio of about eight-to-two.