The Iraqi government has declared an immediate curfew in the capital, Baghdad, to run until Sunday morning.
The curfew affects all vehicles and people on foot
The move affects both vehicles and pedestrians, a spokesman for the Iraqi prime minister said.
A senior Iraqi military spokesman said the curfew had been introduced due to specific intelligence about a series of planned car bomb attacks in Baghdad.
Sectarian violence and bombings have been increasing in the city, thought to coincide with the beginning of Ramadan.
Late on Thursday the brother-in-law of the new chief judge in Saddam Hussein's trial was shot dead.
The curfew order was announced on state broadcaster Iraqiya.
"The government has decided to enforce a curfew on vehicles and individuals starting from Friday evening until 0600 on Sunday morning (0200 GMT)," the message said.
Vehicles have been barred from the Iraqi capital on a number of occasions in recent months.
A three-day curfew was put in place in Baghdad and three provinces in February after the bombing of an important Shia shrine sparked violent protests, but pedestrians were allowed to walk to mosques.
US and Iraqi troops have been reinforcing operations against insurgents and sectarian militias in Baghdad.
On Wednesday a US military spokesman said that there had been a spike in violence coinciding with the start of the Muslim holy month of Ramadan, which began on Monday, and that suicide attacks were at record levels.
Judge Mohammad Oreibi al-Khalifa was appointed last week
He also said Iraqi security forces were "making a concerted effort" to end sectarian violence by targeting death squads.
Shortly after he spoke, 10 people were killed and 11 injured in a shoot-out at a Sunni mosque in the capital.
Shia and Sunni militia groups have increasingly been engaged in tit-for-tat attacks on each other since the al-Askari shrine was bombed in February.
A Sunni-led insurgency that erupted after the ousting of Saddam Hussein is also continuing.
On Thursday, gunman ambushed a car carrying Kadhem Abdul Hussein, the brother-in-law of Judge Mohammad Oreibi al-Khalifa, who is presiding over Saddam Hussein's genocide trial.
Mr Abdul Hussein was killed outright and police said his son was injured.