A powerful roadside bomb went off against a minibus and a taxi in central Baghdad's Nahda area early Tuesday morning, killing nine Iraqis and leaving eight wounded, an interior ministry official said.
The explosion detonated close to a bus station and was followed by two similar roadside blasts believed to be aimed at police patrols.
Three officers were wounded in the two follow-up attacks, which went off within minutes of each other.
The blasts came as the United States boosts its troop levels in the Iraqi capital in an effort to stop insurgent and sectarian violence, which has raised fears of full-blown civil war.
The violence is claiming around 100 lives every day and undermining confidence in Iraq's new Shi'ia-led government.
The government of Nuri al-Maliki, the Iraqi prime minister, has vowed to confront militias, but some of the armed groups have close ties to parties in the administration.
Donald Rumsfeld, the US defence secretary, has extended the tour of duty for about 3,700 troops from the 172nd Stryker Brigade, based in Mosul, so they could be sent to Baghdad. The troops began arriving in the capital on Sunday.
General George Casey, the top US officer in Iraq, told a news conference on Monday that the US and Iraqi troops would drive guerrillas and militia "death squads" from Baghdad and improve security by Ramadan, which falls in late September this year.
But a security crackdown in the capital, ordered by the government and involving about 50,000 US-trained Iraqi security force personnel, has failed to ease bloodshed.
On Monday, at least 28 people were killed and 64 wounded in attacks around the country.