At least 15 people were killed in a spate of insurgent attacks across Iraq a day after the US military lost six marines in the west of the country.
Funerals were held for 19 people found dead on Monday
Four are known to have died and 23 were wounded by a powerful suicide car bomb near a US military convoy in Baghdad.
And gunmen shot dead the head of Abu Ghraib police station, a hospital manager and two ministry officials.
Announcing the marines' deaths, the Pentagon said they were killed in action but did not give details.
One unofficial report says they were a six-man sniper team.
The troops were killed near Haditha, some 200km (125 miles) north-west of Baghdad - an area of Anbar province near the Syrian border that has seen frequent insurgents attacks on US troops.
Insurgents were said to have handed out leaflets in the streets of Haditha claiming they were responsible for the marines' deaths.
Central Baghdad was shaken when a suicide car bomber blew himself up as a US military convoy passed by in Tahrir Square at around 1300 local time (0900 GMT).
At least one US Humvee was set alight and some 14 other vehicles were damaged by the blast, an AFP news agency correspondent said. Shops were also reported to have been badly damaged.
At least four bodies and 23 injured people had been brought in to Ib-Nafis hospital, a doctor there was quoted by the AFP as saying.
"We were 20 metres away when the car bomb went off and me and my team carried 15 wounded civilians," policeman Hussein al-Musawi said.
Elsewhere in Iraq:
- Police chief Mizhir Mohammed Youssef was shot dead by unknown gunmen who ambushed his car in western Baghdad as he drove to work at Abu Ghraib police station, officials said
- Two finance ministry employees were shot dead in Baghdad by gunmen on their way to work, an interior ministry official said
- There were reports of five civilians killed as gunmen ambushed their car elsewhere in Baghdad
- In the town of Baquba, 50km (30 miles) north-east of Baghdad, a car bomb blew up near an Iraqi police patrol, killing a policeman and a child
- The head of Khaluss hospital in Baquba, Dr Abdul Muhsin Ali, was reportedly shot dead by gunmen as he left a health conference.
Funerals have been held for the 19 people whose bodies were found dumped near a school in south-west Baghdad on Monday.
The bodies showed evidence of the victims having been tortured.