Gunmen have attacked a Sunni Arab area of Baghdad, burning mosques and homes, with at least 30 people reported killed, according to police officials.
The victims of Thursday's bombings were being buried during the day
The attacks were in apparent revenge for Thursday's bombings that killed more than 200 people in the Shia Sadr City district of the Iraqi capital.
Fleets of vans left Baghdad to take the coffins of those victims for burial in the ancient Shia holy city of Najaf.
The latest violence came despite a city-wide curfew and appeals for calm.
To add to the Iraqi government's woes, a key Shia group threatened to quit parliament and the cabinet if Prime Minister Nouri Maliki goes ahead with a planned meeting with President Bush next week.
There is a real feeling that the situation is moving to the brink amid the cycle of attacks, says the BBC's David Loyn in Baghdad.
Thursday's bombing and retaliatory attacks were "deplorable", the White House said.
"It is an outrage that these terrorists are targeting innocents in a brazen effort to topple a democratically elected government. These killers will not succeed," White House spokesman Scott Stanzel said.
A Sunni area in the once-mixed Hurriyah neighbourhood came under attack on Friday when gunmen rampaged through the area, setting four mosques and several houses alight.
Iraqi police said some 30 people had been killed, but a defence ministry officials told the French news agency, AFP, that the clashes were so intense that precise information was difficult to obtain.
Clashes also erupted in Sadr City on Friday, where residents said a US helicopter fired on militiamen who were launching rocket attacks.
Violence was also reported in other parts of Baghdad and elsewhere in Iraq, including in the northern town of Talafar where a suspected double suicide bombing killed at least 22 people.
Baghdad has been under an indefinite curfew since Thursday's bombings and the airport remains closed.
The security situation has forced Iraqi President Jalal Talabani to postpone a visit to Tehran on Saturday for talks with his Iranian counterpart Mahmoud Ahmadinejad.
During the curfew, the only people and vehicles officially allowed on the streets of Baghdad were the residents of Sadr City who began the journey to bury their dead.
Thousands of mourners came out onto the streets, walking alongside a seemingly endless fleet of mini-buses, each carrying a coffin on its roof.
The bodies were then driven to an ancient cemetery in the holy city of Najaf, the traditional burial place for Shias, 160km (100 miles) south of Baghdad.
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.
Moqtada Sadr's group is calling for US forces to leave
Sadr City is largely controlled by the Mehdi Army, a Shia Iraqi militias accused of carrying out many sectarian attacks on Sunni areas.
Thursday's bombings could have a deep political impact, with the group led by radical cleric Moqtada Sadr calling on Mr Maliki to call off his planned talks with President Bush.
People in Sadr City faced insurgent attacks as well as repeated raids by US forces, the group said in a statement.
Mr Sadr's followers hold six cabinet posts and have 30 members in the 275-seat parliament.
The withdrawal of the group headed by Mr Sadr would be a major blow to an already unstable government, the BBC's Andy Gallacher in Baghdad says.
The meeting with Mr Maliki is due to take place in Jordan, and a White House spokesman said on Friday there had been no changes to Mr Bush's schedule.