A bomb killed 52 people today at a market in eastern Baghdad's volatile Sadr City slum, police said, six days before U.S. combat troops are due to withdraw from Iraqi towns and cities.

About 104 people were also wounded in the blast, which was one of the worst attacks in Iraq this year. A witness said the explosion tore through a part of the market where birds are sold, setting stalls ablaze.

Violence in Iraq has dropped sharply in the past year, but militants still launch deadly bombings aimed at undermining the government and reigniting sectarian conflict.

Three school students died in another bombing in Sadr City on Monday, one of a string of blasts across the country that killed 27 people that day. In Kirkuk province late last week 73 people were killed in a truck bomb.

Such attacks cast doubt on Iraqi forces' ability to stand alone against a stubborn insurgency.

Sadr City is a bastion of support for fiery anti-American Shi'ite cleric Moqtada al-Sadr and his Mehdi Army militia. But the Mehdi Army has frozen most activities in the past year and Iraqi government forces have retaken control of the area.

Analysts say attacks are likely to intensify ahead of a parliamentary election in January. Iraqi Prime Minister Nuri al-Maliki has built his reputation on reducing the bloodshed, and has lauded the partial withdrawal of U.S. troops.

Maliki, a member of Iraq's Shi'ite majority, has urged Iraqis not to lose heart if insurgents take advantage of the U.S. military drawdown to step up attacks.

U.S. troops who invaded over six years ago are due to leave Iraq completely by 2012 as part of a security agreement signed by Baghdad and Washington last year.