The curfew is from 2pm (1000 GMT) on Friday to 6am on Saturday. There had already been an 11am-3pm ban on traffic in the capital.
It was not immediately known why the government had imposed a curfew, but there has been a security clampdown in Baghdad recently.
Mowaffaq al-Rubaie, the national security adviser, denied media reports that a state of emergency had been declared.
"This is absolutely false. There is no state of emergency. It is only a curfew," Rubaie said.
He would not say whether the curfew was imposed after clashes broke out in central Baghdad. No deaths were reported.
Show of force
Dhafir al-Ani, spokesman for the Iraqi Accordance Front, said: "Large numbers of (Shia) al-Mahdi militia have attacked Sunni districts in Baghdad, in a clear show of force.
"They faced fierce resistance from the armed inhabitants. We urge the Iraqi army, who's in control of the attacked areas, to take responsibility and put a limit to such provoking acts."
Al-Ani urged Iraqi army to curb
attacks on Sunni neighbourhoods
Al-Ani blamed the clashes on the extended curfew. The clashes took place in Haifa street, Utafiya and al-Fadil districts in central Baghdad.
Al-Ani said clashes had been going on between fighters and US forces and Iraqi police until Friday afternoon.
Also on Friday, at least 22 people were killed in two bombings, including one directed at a Sunni mosque at the time of Friday prayers.
At least 12 people were killed and 20 others wounded when the bomb went off outside the Hibhib al-Kabir mosque in the village of Hibhib, north of Baghdad, as worshippers were just finishing their Friday prayers, police said.
The bomb was planted close to the rear door of the mosque, police said, adding that the front gate of the mosque was closed for security reasons.
Site of al-Zarqawi hit
Hibhib, close to Baquba, hit world headlines after US forces killed al-Qaeda in Iraq leader Abu Musab al-Zarqawi in the village on June 7.
In Baghdad's Dura district a policeman was killed and four wounded when a roadside bomb exploded near their patrol.
The latest attacks come despite nearly 60,000 US and Iraqi troops patrolling the streets since June 14.
Four members of the al-Mahdi Army militia were killed in Haifa street as they were guarding Shia praying in Baratha Shia mosque in central Baghdad.
After the mosque was attacked last Friday, Muqtada al-Sadr, the leader of the al-Mahdi Army, called on his followers to hold Friday prayers in Baratha to show solidarity with the victims.
Basra car bomb
Elsewhere, at least 10 people were killed and 18 others wounded in a car bomb attack in Iraq's southern city of Basra on Friday, police said.
Police said the bomber's car exploded near a service station in the centre of Basra city about 12pm.
The British military, whose forces are based in southern Iraq, said it had no information on the attack.
Basra has been the site of a militia war that prompted Nuri al-Maliki, the prime minister, to declare a state of emergency in the oil-rich governorate which has Iraq's only access to the Gulf.