"The operations termed 'Cutting Sword' have been launched in Tikrit from Daash," a security official told Xinhua.
Army and police forces backed by tanks and helicopters moved into the city from several routes at dawn.
Fierce clashes broke out with Sunni militants from the Islamic State (IS) terrorist group, formerly known as Islamic State of Iraq and Greater Syria (ISIS).
"They entered the central part of Tikrit and are fighting to free the rest of the city," the official added.
Heavy fighting still continues as the troops are moving from the south of Tikrit toward the provincial government building in the central part of the city, while other troops moved from al-Qadsiyah neighbourhood in the north towards downtown Tikrit.
The offensive came after several failed attempts by the Iraqi security forces to recapture Tikrit, some 170 km north of Iraq's capital Baghdad.
The seizure of Tikrit, hometown of deposed president Saddam Hussein, June 11 was part of deterioration in security which started in Iraq's northern province of Nineveh earlier this month when bloody clashes broke out between security forces and Sunni militants.