The gunmen, wearing army uniforms, took people inside Salahuddin’s provincial council building hostage, police said.
A Reuters correspondent at the scene, 150 km (95 miles) north of Baghdad, said Iraqi security forces and U.S. soldiers had surrounded the building.
The attack in Tikrit, which lasted more than five hours, killed at least 55 people and wounded at least 96 others, including top police officials, three provincial council members and three journalists.
According to an Interior Ministry official, the attack on the provincial government building was carried out by an unknown number of men dressed in Iraqi army and police uniforms.
They fired mortars before overcoming the security force and entering the building. Top provincial police officials were killed, and a number of others were taken hostage by the attackers, the official said.
A car bomb near the entrance was detonated as other security forces arrived.
Nearby U.S. forces responded to the initial incident but did not get directly involved, according to Col. Barry Johnson, a U.S. military spokesman.
“Our assistance has been limited to providing aerial surveillance of the scene and keeping our soldiers on site to receive further requests for assistance if needed,” Col. Johnson added.
The siege ended when Iraqi forces entered the building, supported by U.S. helicopters, and killed all the insurgents, the Interior Ministry official said.
The northern provinces of Salahaddin, Diyala and Ninewa continue to be a place of frequent attacks by insurgents, including those linked to al Qaeda in Iraq. No one has yet claimed responsibility for the siege in Tikrit, the hometown of former Iraqi dictator Saddam Hussein