(Adds comment from police official paragraphs 8-9)

BAGHDAD, March 8 (Reuters) - A suicide bomber killed 28 people and wounded 57 on Sunday at the main police academy in Baghdad, the first major attack in almost a month in the Iraqi capital.

Many police and police recruits were among those killed when the bomber, wearing an explosive vest and riding a motorbike also packed with explosives, blew himself up at the back entrance of the police academy in central Baghdad, police said.

Body parts were scattered at the scene and police struggled to determine the identities of the victims.

Violence has dropped sharply in Iraq from the height of sectarian and insurgent bloodshed unleashed by the 2003 U.S.-led invasion to topple Saddam Hussein.

But the country remains a dangerous place, and areas such as the northern city of Mosul are still in the grip of a stubborn insurgency. A car bomb in a livestock market in southern Iraq killed 12 people on Thursday.

On Feb. 11, 16 people were killed 25 wounded when twin car bombs exploded at a bus terminal and market area in Baghdad.

Police recruits have been a major target for militant attacks in the past. On Dec. 1 last year, an attack killed 15 policemen and recruits and wounded 45 other people outside the same Baghdad police academy.

"We know recruits are a favourite target for suicide bombers. We tell them to come in small groups instead of big groups, but they don't pay attention," an academy official said.

"This is the result -- a suicide bomber managed to infiltrate and explode himself," he said, requesting anonymity. Iraq has expanded the ranks of its police and military forces by hundreds of thousands of men in recent years as the government of Prime Minister Nuri al-Maliki seeks to ensure local forces can provide security, with U.S. forces preparing to end combat operations by September 2010.

Under U.S. President Barack Obama's new plan, up to 50,000 U.S. soldiers would stay in Iraq to train and equip local forces, protect civilian reconstruction projects and conduct limited counter-terrorism operations until all U.S. forces are required to leave by the end of 2011.

U.S. and Iraqi forces acknowledge that Iraqi forces are in urgent need of equipment and specialised training before they can take over sole responsibility.

Air support and logistics are two areas in which Iraqi forces are seen as especially weak. (Reporting by Ahmed Rasheed)

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