Funerals are being held in Iraq for victims of Thursday's bomb attacks in Baghdad's Shia Sadr City district that left more than 200 people dead.
In apparent retaliation, mosques were attacked in a Sunni area of Baghdad with unconfirmed reports of casualties.
The latest violence came despite a city-wide curfew and appeals for calm.
A key Shia group blamed US troops for the violence in Sadr City, saying it will quit the government if PM Nouri Maliki meets President Bush as planned.
The withdrawal of the group headed by radical Shia cleric air Moqtada Sadr would be a major blow to an already unstable government, the BBC's Andy Gallacher in Baghdad says.
Baghdad is under an indefinite curfew, but fresh violence broke out on Friday.
Gunmen attacked four mosques in a Sunni area in the Hurriya neighbourhood, Iraqi officials said.
The most serious damage happened soon after Friday prayers when a mosque was burned down. A rocket-propelled grenade exploded inside another, while two others were sprayed with bullets fired from AK-47 assault rifles.
Local people said there had been casualties and homes were still on fire.
There has been no official confirmation but a defence ministry officials told the French news agency, AFP, that the clashes were so intense that precise information was difficult to obtain.
The only people and vehicles officially allowed on the streets were those taking part in the funeral processions.
Mourners cried as they walked beside vehicles taking coffins south to Najaf, the traditional burial place for Shias, which is 160km (100 miles) south of Baghdad.
Thousands of men, women and children beat their chests, chanted and cried as they accompanied the processions.
Bodies have been arriving in Najaf to be buried in the city's ancient cemetery.
Show of unity
Thursday's multiple car bomb attacks in Sadr City - in which 250 people were also wounded - were the deadliest in Iraq since the US-led invasion of 2003.
Leaders of Iraq's Shia, Sunni and Kurdish communities appealed for calm in a show of unity.
BLOODIEST DAYS OF VIOLENCE
23 Nov 2006 - 202 dead
Wave of car bomb and mortar blasts strike Sadr City in Baghdad
7 April 2006 - 85 dead
Triple suicide bombing at Shia Buratha mosque in Baghdad
5 Jan 2006 - 110 dead
Suicide bombers hit Karbala shrine and police recruiting station in Ramadi
14 Sept 2005 - 182 dead
Suicide car bomber targets Baghdad labourers in worst of a series of bombs
28 Feb 2005 - 114 dead
Suicide car bomb hits government jobseekers in Hilla
2 March 2004 - 140 dead
Suicide bombers attack Shia festival-goers in Karbala and Baghdad
1 Feb 2004 - 105 dead
Twin attacks on Kurdish parties' offices in Irbil
Iraq's most prominent Shia cleric, Grand Ayatollah Ali Sistani, urged people "not to react illegally and maintain self-restraint", one of his officials said.
The prime minister also called on Iraqis not to resort to violence.
"We denounce sectarian practices that aim to destroy the unity of the nation," Mr Maliki said in a television broadcast on Thursday.
Sadr City is largely controlled by the Mehdi Army, the best-known of the Shia Iraqi militias, which has been accused of carrying out many sectarian attacks.
Thursday's bombings could have a deep political impact with the group led by Mr Sadr threatening to quit the unity government.
"We will withdraw from the government and parliament if the prime minister meets Bush," the group, which blames US-led forces for the violence, said on Friday.
The meeting is due to take place in Jordan next week.
Mr Sadr's followers hold six cabinet posts and have 30 members in the 275-seat parliament.
Elsewhere in Iraq, at least 22 people were killed by a suspected double suicide bombing in a market in the northern city of Talafar, police said.
Talafar, close to the Syrian border, was once a bastion for Sunni guerrillas, but has been held up by US forces in Iraq as a example of successful counter-insurgency operations.