The blast happened at around 7:00pm in the busy business district of Hay al-Amel in the west of the city, he said.
There was no immediate claim of responsibility for the explosion, which bore all the hallmarks of a suicide attack by the Islamic State group.
Fighting Daesh: a test for US-Iraq relations?
The radical extremist group is under assault in both Iraq - in the country's second city Mosul which IS seized in June 2014 - and in neighbouring Syria.
In that year, the militants took vast swathes of Iraqi territory north and west of the capital.
Iraqi government forces backed by the US-led international coalition have since retaken many cities, including Tikrit.
But as IS has lost ground in Iraq, it has also retained the ability to stage regular attacks there.
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