Police in the Pakistani city of Karachi have fired tear gas at thousands of angry mourners after an attack on a Shia mosque killed at least 20.
Clashes brought the city to a standstill
Trouble erupted after funeral prayers for 14 of those killed in Monday's attack, which officials believe was a sectarian suicide bombing.
The funerals follow overnight unrest in which three people died in clashes with the police.
President Musharraf has pledged action to deal with the sectarian violence.
Tuesday's clashes began in the city centre, but disturbances were reported from other parts of the southern port.
Violence started when the funeral procession made its way towards the graveyard where victims were to be buried, and a section of the Shia youth clashed with riot police.
Protesters set fire to several cars and motorcycles, and torched a number of shops before paramilitary troops moved in to disperse them.
Thousands of troops and paramilitary forces are on maximum alert to prevent further clashes between extreme Shia and Sunni factions in Karachi.
The latest violence followed Monday's mosque attack, which also injured another 40 people.
PAKISTAN'S SECTARIAN DIVIDE
Shias revere Ali, son-in-law of the Prophet Mohammed
Pakistan is 20% Shia, 70% Sunni
Violence between Sunni and Shia factions began from early 1980s
Over 150 people have died in the past year alone
Around 4,000 people have been killed in total
Most violence takes place in Sindh, Balochistan and Punjab
It was believed to have been triggered by the killing of the senior Sunni cleric, Mufti Nizamuddin Shamzai, over the weekend by unidentified gunmen.
The BBC's Zaffar Abbas in Islamabad says Karachi has a long history of religious and ethnic violence, but the month of May was the worst in recent years - with more than 50 people killed in different incidents of violence.
President Musharraf said he would take tough measures to restore order in the city - but did not specify what they would be.
"I will take serious action, this is the second incident
within 24 hours, following Mufti Shamzai's killing," Information Minister Sheikh Rashid Ahmed quoted the president as saying.
The minister denied the authorities had lost control of Karachi.
Sindh police chief Kamal Shah described the violence as a war between the militants and the police.
"Police are after them and they are after the police. Many of the terrorists have been arrested but it is an ongoing war against terror," he said.
Senior Shia leader Hasan Turabi has meanwhile appealed for calm in the aftermath of the violence, which he said was the worst since 1984 and was intended to divide the two main Muslim factions.
"There is no Shia-Sunni conflict and we will never allow the two sects to fight against each other," he said.
Sunni-Shia sectarian violence has killed as many as 4,000 people in the past 15 years in Pakistan.
Monday's explosion was so powerful it knocked down a pillar and one of the side walls of the mosque.
An office building belonging to the trust that manages the mosque also collapsed.
An emergency was declared at three hospitals and appeals were made for blood donations.
A night of violence followed the attack
Police had been trying specifically to protect Shia mosques following Mufti Shamzai's death.
The attack on his car came three weeks after at least 14 people were killed in Karachi when a man, apparently dressed as a Shia cleric, blew himself up in a Shia mosque.
The police have accused militants of deliberately trying to foment sectarian violence following the recent arrests of hard-line militants.
The authorities were investigating reports that a person pretending to be a worshipper had left a briefcase in the mosque shortly before Monday's explosion.
But police are now saying there is a strong possibility the blast was caused by a suicide attack.
"We did not find any crater in the mosque, which shows that it was a suicide attack," senior police investigator Manzoor Mughal told the Associated Press news agency.