Two explosions in a central Baghdad district have killed at least 27 people, Iraqi police say.
The car bomb caused substantial damage to buildings
The blasts happened in the commercial district of Karrada, a mostly Shia area. At least 101 people were injured.
A car bomb went off near a restaurant popular with Iraqi police. Then several mortars exploded near a petrol station, damaging nearby buildings.
The attacks shattered the relative peace which fell on the city on Wednesday, says the BBC's Jane Peel.
Many of the injured were said to be in a serious condition.
Karrada - a largely peaceful district populated mainly by Shia Muslims - is also home to Sunnis and Christians who mingle together in the local restaurants and coffee shops.
It is also well protected with a heavy presence of Iraqi troops and several checkpoints, our correspondent says.
Separately, there are reports that gunmen have killed four security guards outside a Sunni mosque in western Baghdad.
On Tuesday, President Bush announced that US troops would be deployed to the Iraqi capital to deal with the deteriorating security situation.
The deployments would "better reflect the current conditions on the ground", he said at a joint news conference in Washington with Iraqi Prime Minister Nouri Maliki.
The troops would help to secure suburbs where militants operated, he said.
An average of more than 100 civilians per day were killed in Iraqi violence in May and June, according to the UN.
The security plan was the second announced for Baghdad in the past six weeks.
Iraq has experienced an upsurge in sectarian violence in recent months.
Last Sunday, at least 34 people were killed in a blast in Sadr City - a mainly Shia area in the capital.