The blast, which also wounded 150, was the first major strike in Iraq since the formation of a new government on December 21 and recalled an August attack against an army recruitment centre in Baghdad that also killed dozens.
"I have been trying for hours to call my brother, he was in the queue to join the police but his phone is off," said Mohammed Aiseh, who was standing at a checkpoint set up to bar family members from entering the city's hospital, which was already filled with victims.
"I don't even know if he is dead or wounded," the 38-year-old said, sobbing.
An AFP journalist said the bomb site in the middle of Tikrit, the former hometown of now-executed dictator Saddam Hussein, 160 kilometres north of the Iraqi capital, was covered in torn off flesh and pools of blood, with pieces of clothing and shoes scattered across the scene.
Policemen and soldiers had cordoned off the blast site and several ambulances were rushing wounded people to a nearby hospital.
"Fifty people were killed and 150 wounded by a suicide bomber at a police recruitment centre in Tikrit," an interior ministry official said in Baghdad, speaking on condition of anonymity.
A police officer in Tikrit said the majority of those killed were potential recruits.
Salaheddin provincial council, which held an emergency session following the blast, called on central government authorities in Baghdad to recognise the dead as members of the police, so that their families would be awarded higher levels of compensation.
It also declared three days of mourning and said all the wounded would be allowed to join the police.
The interior ministry official said that some of the wounded would be transferred to facilities as far away as Baghdad and the northern cities of Kirkuk and Mosul because Tikrit's main hospital was not able to cope.
"The number of wounded is too high," said provincial police chief General Raad al-Juburi. "We have transferred many of them outside of Tikrit."
Mosques in Tikrit were, meanwhile, using loudspeakers to broadcast calls for people in the city to donate blood to hospitals.
Witnesses, who declined to be identified, said the recruits had been queuing to enter the centre since 6:00am, with the attacker detonating his payload at the entrance to the site at around 07:15 GMT.
The death toll was the highest from a single attack since Al-Qaeda insurgents stormed a Baghdad church on 31 October with the ensuing siege leading to the deaths of 44 worshippers, two priests and seven security force members.
It was also the first major strike in Iraq since Prime Minister Nouri Al-Maliki named a new cabinet on 21 December, ending nine months of stalemate after the 7 March elections.
Insurgents have targeted Iraqi security recruitment centres in the past. A suicide bomber blew himself up in a crowded army recruitment centre in Baghdad on 17 August, killing 59 people and wounding 125.
Iraq's security forces are now solely responsible for the country's security, with the United States having declared a formal end to combat operations in the country at the end of August.
While the US military still has around 50,000 soldiers stationed in the country, they are tasked mostly with training and advising their Iraqi counterparts, and must withdraw completely by the end of the year.
Violence across Iraq has declined substantially since its peak in 2006 and 2007, but attacks remain common. The total death toll for last month was the lowest since November 2009 and marked the fifth month in a row in which the death toll has been lower than the previous month.