"Mechanical shovels have been brought in to lever up the wreckage that has fallen, to rescue the dead and wounded. The rescue operation is continuing, and Iraqi and US forces are at the scene," Baz said.
Dawr's municipal offices and post office were also damaged in the blast.
"The kamikaze parked a truck carrying animal feed against the wall of the station then blew it up. The majority of the building is demolished," Hammad al-Duri said.
A second, smaller roadside bomb exploded further north in Tikrit itself, killing two bystanders, Mazen al-Juburi, Iraqi police captain, said.
Further north near the Syrian border, armed men attacked a bus carrying new recruits from the Iraqi border security force.
The guards had signed up in the border town of Rabiaa and were heading south to their new base when the attackers fired automatic weapons at their minibus, killing all eight men aboard, Jassim Mohammed, a regional official told the AFP news agency.
It was not immediately clear which of Iraq's many armed groups was behind the attacks, but police are regularly targeted by Sunni-led fighters opposed to Iraq's Shia-led government and its US sponsors.