A suicide bomber blew himself up in a crowd of police recruits on Tuesday, killing at least 50 people and undercutting Iraqi security efforts as the nation struggles to show it can protect itself without foreign help.

The death toll was still rising more than three hours after police said the bomber joined a crowd of more than 100 recruits and detonated his explosives-packed vest outside the police station in Saddam Hussein's hometown of Tikrit, some 130 km north of Baghdad.

One recruit who survived the blast said the jobseekers were frisked before they entered the station's yard.

“We were waiting in the line to enter the police station yard after being searched when a powerful explosion threw me to the ground,” said recruit Quteiba Muhsin, whose legs were fractured in the blast. “I saw the dead bodies of two friends who were in the line. I am still in shock because of the explosion and the scene of my two dead friends.”

Loudspeakers from the city's mosques were calling on people to donate blood for the wounded. An Iraqi television station broadcast footage from the scene that showed pools of blood, bits of clothing and shoes of the victims scattered near a concrete blast wall.

Tikrit is the capital of Sunni-dominated Salahuddin province, and the city sheltered some of Al-Qaeda's most fervent support after the 2003 U.S.-led invasion ousted Saddam.

Insurgents have long found recruitment centres a favourite target, taking advantage of lax security measures just outside protective barriers at police stations and the confusion caused by desperate jobseekers scrambling for work in a country with an unemployment rate as high as 30 per cent.

A similar strike on an Iraqi recruitment centre and army headquarters in central Baghdad last August left 61 dead and 125 wounded in what was one of the deadliest attacks of the summer.