Shia militias clashed with Iraqi and US forces in Baghdad and 10 worshippers died in a bomb blast outside a Sunni mosque in the village where a US air strike killed Abu Musab al-Zarqawi this month.
Iraqi Prime Minister Nuri al-Maliki slapped a sudden curfew on the capital after the clashes broke out there but lifted the ban on vehicles and people hours later.
In Iraq's second city of Basra, a car bomb killed at least five people at a petrol station, hospital sources said. Earlier, police said 10 had died in the blast, which came despite a state of emergency declared last month to end militia violence.
Basra's governor said only two people were killed. Politicians in the south have contested hospital death tolls in the past.
Underscoring the challenges Maliki faces in easing violence that has pushed Iraq to the brink of civil war, militias loyal to radical Shiacleric Moqtada al-Sadr exchanged fire with gunmen in the former Sunni insurgent stronghold of Haifa Street.
Iraqi and US forces who rushed to the area became involved in the fighting, police sources said.
A Reuters cameraman saw militiamen from Sadr's Mehdi Army running across the street 200 metres (yards) from a US Humvee. Sadr, whose followers are members of the ruling ShiaAlliance, has staged two revolts against US and Iraqi troops.
As mortar bombs crashed and gunfire crackled in central Baghdad, Maliki, who took office last month, imposed a curfew banning people and cars from the streets from 2 pm until 6 am on Saturday. It was lifted two hours after it was declared, Maliki's office said.
State television quoted Gen. Abdul Aziz Mohammed, operations chief at the Defence Ministry, as saying the curfew was a response to the fighting. Three Iraqi policemen and five Iraqi army soldiers were wounded. The violence later subsided.
There was no immediate comment from the US military, which launched a sweep last year to root out rebels in Haifa Street.
The bomb outside the mosque went off in Hibhib, north of Baghdad, where Zarqawi, leader of al Qaeda in Iraq, was killed on June 7.