A daytime curfew is in force in and around the Iraqi capital, Baghdad, in an attempt to curb a surge in violence.
Police have been ordered to seize any private vehicles that defy the ban, on what is the Muslim day of prayers.
In continuing violence, the bodies of at least 18 people killed in a suspected sectarian attack were found near Baghdad on Friday.
Hundreds of people have died since a key Shia mosque was bombed last week in the city of Samarra.
In the latest large-scale attack, police quoted residents as saying about 50 gunmen ambushed the small town of Nahrawan, south-east of Baghdad, at nightfall on Thursday.
They said the attackers targeted a power station, killing an as-yet unknown number of people, before moving on to two brick factories, where they killed 18 workers, all believed to be Shia.
Last week, the bodies of 47 factory workers, who had been dragged from their vehicles and shot, were also found in Nahrawan.
Prime Minister Ibrahim Jaafari said he had ordered the curfew "because of the sensitive security situation our beloved country is passing through".
Friday's ban - which came into effect as the regular overnight curfew ended - will remain in place until 1600 (1300 GMT).
Although cars are banned from the streets, people will still be able to walk to nearby mosques for Friday prayers.
Mr Jaafari urged mosque leaders not to use "inflammatory" language in their sermons on Friday, warning of "severe measures" if they try to "incite terrorism".
"The street is angry and they should know how to calm the people," he told reporters.
A similar curfew was imposed last weekend to try to quell a wave of sectarian violence that followed the bombing of the shrine in Samarra on 22 February.
At least 400 people have been killed.
On Thursday, at least nine security forces members died in an attack on a checkpoint near Tikrit.
Mr Jaafari cancelled a meeting with senior political leaders on Thursday, apparently to protest against a campaign to oust him.
Kurdish and Sunni Arab leaders are unhappy with Mr Jaafari, and have said they will not join a national unity government with him at its head.
It is the latest crisis to hit attempts to form a new government following the December election.