The curfew bans all vehicles from moving along the streets of Baghdad and nearby Baquba - where al-Zarqawi was killed - between 11am (0700 GMT) and 5pm, coinciding with Friday prayers.
An Iraqi defence ministry official said: "The curfew is a measure to keep people in doors as there could be more bombings like the ones last night, following al-Zarqawi's death."
Three bombs killed 26 people in predominantly Shia areas of Baghdad on Thursday.
In other developments, a senior US military spokesman has disclosed that a mortally wounded al-Zarqawi, still alive after a US air strike on his hideout, mumbled briefly and attempted to "turn away off the stretcher" he had been placed on by Iraqi police.
But he was dead when US soldiers arrived on the scene, the officer said.
Major-General Bill Caldwell said on Friday he learned after getting briefings on Friday that al-Zarqawi was alive when Iraqi police first arrived on the scene, but he died a short time later.
He said: "We did in fact see him alive. He mumbled a little something but it was indistinguishable and it was very short."
Caldwell also said that a series of US military raids based on intelligence gathered from the destroyed hideout had yielded at least two dozen captives and a hidden cache of weaponry, Iraqi army uniforms and other materials.
A curfew bans all motorists on
Baghdad and Baquba's streets
Speaking to reporters at the US Defence Department by video-teleconference from his office in Baghdad, he said that after 17 such raids in the immediate aftermath of Wednesday's air strike on al-Zarqawi's safehouse, more were launched on Thursday.
Caldwell displayed digital photographs of recovered items that he said included a suicide belt, a flak vest, passports and identification cards, vehicle licence plates, ammunition belts, rifles and other guns and a night-vision device.
He said they were found under the floorboards of a building; he did not identify the location.
Two days after al-Zarqawi's killing, murders and kidnappings show no signs of tapering off.
In a daring operation, armed men kidnapped Muthanna al-Badri, director-general of state company for oil projects, or SCOP, while he drove on Thursday in a predominantly Sunni Arab neighbourhood of Baghdad, ministry spokesman Assem Jihad said.
On the same day, Sunni cleric Riyadh Shelal Al-Khafaji was gunned down by men in black uniforms commonly worn by Shia militias in Iskandariyah, 50km south of Baghdad.
And on Friday, a firefight west of Baquba killed five civilians and wounded three, and demolished five houses. The circumstances of their deaths were unclear.
Also on Friday, the torso of a man wearing a military uniform was found floating in a river near Kut, southeast of Baghdad. Seven other bullet-riddled bodies were found floating elsewhere in the area.
Meanwhile, in Iraq, ordinary people have reacted largely along sectarian lines to the news of al-Zarqawi's death.
He was mourned in Anbar province, the heart of the Sunni-led insurgency.
Abid al-Dulaimi, 40, said: "This is a black day in Ramadi. This a great loss for all the Sunnis.
"If they killed al-Zarqawi, more than one al-Zarqawi will come out."
Al-Zarqawi was reported to be
alive when Iraqi police found him
In the Shia Sadr City area of Baghdad, there was a positive view.
Anwar Abdul Hussein, a baker, said: "We hope the killing of al-Zarqawi and his aides, those who killed many Iraqis, will finish all the terror in Iraq and let everybody live safely."
While many Iraqis welcomed the news of his death, they also expressed scepticism that it would stem the sectarian and militia violence taking place across the country.