Dozens killed in overnight IS attack on Iraq's Tikrit
The militants wore police uniforms and used a police car to enter the city, a police colonel told Reuters, adding that there were around ten attackers, including at least two suicide bombers.
Separately, provincial council member Ahmed al-Karim told AP the suicide bombers targeted a police patrol and broke into a police officer's house in Tikrit's al-Zhihoor neighbourhood, killing him and four members of his family.
Security forces managed to kill three of the bombers while two detonated their vests as police officers surrounded them.
At least 31 bodies were taken to hospital, including 14 belonging to police officers, Nawaf Mustafa, a doctor in the city's central hospital told Reuters.
Five IS militants are believed to remain at large as security forces in Tikrit declared a curfew on Wednesday.
Military and federal police force arrived early on Wednesday to the city to carry out a search, closing off a number of roads.
"Members of the Popular Mobilisation Forces were also deployed across the city," an interior ministry source told The New Arab.
Iraqi forces drove out IS militants from Tikrit in April 2015. Since then, IS militants have launched a number of big attacks in and around Tikrit, located about 80 miles (130 kilometres) north of Baghdad.
IS has suffered a string of defeats over the past two years – most recently in the Mosul operation where Iraqi forces are battling to drive the extremists out of the western part of the city, a more densely populated area.
Eastern Mosul, which is separated by the Tigris river from the western part of the city, was declared liberated from IS in January.
Despite the severe setbacks, IS has managed to regularly launch attacks in some Iraqi cities.
A series of large-scale bombings claimed by the militant group has also struck Baghdad since the operation to retake Mosul began in October.
Iraqi and US-led international coalition officials have repeatedly warned that after Mosul, IS will likely return to its insurgent roots as it loses more territory in both Iraq and neighbouring Syria.
Agencies contributed to this report