At least 30 others were wounded in the attack, police said.
The attack came as US reinforcements rolled into some of the most violent districts of Baghdad to quell the raging sectarian strife that has left hundreds dead in the Iraqi capital despite a massive security crackdown.
The bomber arrived at a hall located in the centre of Tikrit where people were gathered to mourn the death of the father of provincial council member Saab Abd Badaywi.
He parked an explosive-laden car outside the building and then entered the hall where he blew himself up, a police officer said.
The toll was expected to rise as many victims were caught in the debris after the roof of the hall caved in following the explosion.
The rescue effort was hampered because the bomber's car was outside the building.
Two policemen were killed and five others wounded when their patrol was ambushed by gunmen in southern Baghdad, while earlier in the day two roadside bombs echoed around the city.
Also security forces recovered 20 corpses across Baghdad - four Iraqi soldiers and 16 civilians who had been tortured and shot dead, police said.
An interior ministry official said that clashes between Iraqi security forces and the Shia Mehdi militia loyal to Muqtada al-Sadr had also erupted on Sunday in Baghdad's Sadr City district.
The official said no immediate toll was available from the clashes between Sadr militia and the Iraqi forces.