The gunmen had set off two car bombs outside the provincial council's headquarters and exchanged fire with security units before storming the building and taking an unknown number of people, including elected officials, hostage.
Among those killed in the aftermath were four generals of the military police and a number of local officials and civilians, police said.
An Al Arabiya correspondent was also killed in the attack, the Arabic broadcaster reported.
It was unclear how many of the 10 gunmen were killed in the attempt to free the hostages.
No group has claimed responsibility for the attack. Witnesses said Salah el-Din province, where the city of Tikrit is located, had been relatively calm the past several weeks until Tuesday's bloody hostage-taking.
There are still people missing from the attack, with relatives searching for their family members in nearby hospitals in the Salah el-Din province. Security sources are working to compile an exact death toll.
A military curfew was imposed on the Salah el- Din province, which is 175 kilometres north of Baghdad.
A gun battle ensued for hours between security forces and gunmen before security stormed the building to free the hostages.
A security source who initially said the US military was involved in the exchange of gunfire with the hostage takers later clarified that the US military was only assisting Iraqi security forces logistically.
The US military has not yet commented on the attack nor made any statements about how they assisted the security forces.
Also Tuesday, unknown attackers fired two Katyusha rockets on the Hotel Ambassador, on the bank of the Tigris River as tribal chiefs were meeting there. Three people, including two security guards, were injured and the hotel suffered major damages, the news agency Aswat al-Iraq reported.