More than 160 people have been killed since Tuesday, when troops stormed a Sunni protest camp near Kirkuk, triggering clashes that quickly spread to other Sunni areas in western and northern provinces.
Although well below the heights of 2006-7, this week's violence was the most widespread since US troops pulled out of Iraq in December 2011. Militant attacks have increased this year as Iraq's fragile ethnic and sectarian balance comes under growing strain from the civil war in neighbouring Syria.
In and around Baghdad, eight people including a soldier were killed in a series of bomb blasts outside mostly Sunni mosques.
Later on Friday, a car bomb killed seven in a busy shopping area in the south of the city. In the capital's Shi'ite stronghold of Sadr City, a motorcycle bomb exploded near a kiosk selling falafel, killing five.
No group claimed responsibility for any of the attacks, but Iraq is home to a number of insurgent groups including a local wing of Al Qaeda.
"I call on the conscience of all religious and political leaders not to let anger win over peace, and to use their wisdom, because the country is at a crossroads," UN envoy Martin Kobler said in a statement.
Tens of thousands of Sunni Muslims poured onto the streets of Ramadi and Falluja in the western province of Anbar following Friday prayers, in their biggest show of strength since the outbreak of protests last year.
In Ramadi the preacher, who wore military fatigues with his cleric's turban, gave security forces 24 hours to quit the city, warning he would not be responsible for whatever happened after that.
Sunnis have been protesting since December against what they see as the marginalisation of their sect since the US-led invasion overthrew dictator Saddam Hussein in 2003 and empowered majority Shi'ites through the ballot box.
The demonstrations had recently eased, but this week's army raid on a protest camp in Hawija, near Kirkuk, 170 km (100 miles) north of Baghdad, reignited Sunni discontent and appears to have given fresh momentum to insurgents.
The army entered the town of Suleiman Pek after militants who seized control on Wednesday pulled back in agreement with the security forces and the provincial governor.
"We withdrew from these places in order to avoid bloodletting of our people because we know that the army wants to commit a new massacre similar to what happened in Hawija," tribal leader Jamil Al-Saqr told Reuters.
A tribal leader in the nearby town of Tuz Khurmato later said five bodies had been brought to the hospital, accusing government troops of executing them. An army source denied that.