Baghdad's streets are almost deserted after Iraq's government put the capital and three provinces under curfew to halt sectarian violence.
Security forces have been on the streets to enforce the curfew
At least 130 people - mostly Sunnis - have died since the al-Askari shrine, holy to Shias, was bombed on Wednesday.
In protest at the unrest, Sunni politicians have pulled out of emergency talks with the government.
The curfew began on Thursday evening and will last until late afternoon on Friday, the Muslim day of prayer.
It left the centre of Baghdad eerily quiet on Friday morning, the BBC's Jon Brain in the city says.
Hundreds of police officers are manning checkpoints and turning back motorists attempting to cross the city.
However, people are being allowed to walk to mosques in their local areas.
Many Iraqis were unaware of the extended curfew, which was announced only at midnight - a clear indication that this was a panic measure taken by a government struggling to keep up with events, our correspondent adds.
The curfew, encompassing Baghdad and the provinces of Diyala, Babil and Salahuddine, was due to end at 1600 (1300 GMT) on Friday.
It is expected to be re-imposed from Friday evening until Saturday morning. All police and army leave has been cancelled.
There was some violence overnight despite the curfew, including the reported killing of at least three Shias by gunmen who stormed their house in the town of Latifiya, south of Baghdad.
Twelve bodies were reported to have been found overnight in Baghdad.
In other developments, the US military said it had killed al-Qaeda's leader in northern Iraq, Abu Asma.
A military statement described the man - also known as Abu Anas and Akram Mahmud al-Mushhadani - as an explosives expert with close ties to important car-bomb manufacturers in Baghdad.
Attempt to contain
As attempts continued to ease the sectarian tensions, spokesman for President Jalal Talabani said the Iraqi leadership was united in calling for calm.
"Everything will fall apart if we have civil war so, no, I do not think that there will be an all-out civil war," Hiwa Osman told BBC Radio 4's Today programme.
The streets have been largely empty in areas under curfew
"There might be a few incidents here and there. But the government is now trying very hard, with the help of the multi-national forces, to contain the situation."
A leading Sunni imam, Hasan al-Taha, denounced Wednesday's attack on the Shia shrine in his sermon on Friday at Baghdad's Abu Hanifa mosque.
And there were calls for joint Sunni-Shia prayers at the site of the attack and in Basra, which is not under curfew.
But the Association of Muslim Scholars - the main Sunni religious authority - said at least 168 Sunni mosques had been attacked across the country since the bombing on Wednesday, the Associated Press reported.
The government could confirm only that 19 had been attacked in Baghdad.
Our correspondent says the latest violence has been shocking even by Iraq's standards.
In the worst single incident, 47 people were killed and their bodies dumped in a ditch outside the capital. Both Sunnis and Shias were among the victims.
The main Sunni alliance said it was pulling out of the emergency talks convened by President Talabani after the string of attacks on Sunnis.
It has also announced its withdrawal from negotiations to form a coalition government - a development which could have far-reaching consequences, our correspondent says.
In a rare public rebuke, the Association of Muslim Scholars accused Iraq's top Shia cleric, Grand Ayatollah Ali al-Sistani, of fomenting the violence.
Ayatollah Sistani has urged Shias not to attack Sunni mosques, but a spokesman for the cleric said anger might be hard to contain.
On Thursday radical Shia cleric Moqtada Sadr added his voice to those calling for restraint.
"The occupation is sowing sedition among us," he said.
"Do not allow this to weaken your determination, unity and solidarity."