"A suicide bomber entered the recruiting centre and blew himself up, killing 45 people and wounding 150," provincial governor Nawzad Hadi was quoted as saying.
"We will continue fighting terrorists until we root them out. They will not scare us," he said.
Sunni-backed insurgents have stepped up attacks on Iraqi police and army targets in recent weeks.
25 die in attack on Iraq funeral
Wednesday's attack coincided with the swearing in of the new government led by Prime Minister Ibrahim Jaafari.
But key Cabinet posts like the defence and oil ministries remained unfilled due to wrangling among the Shias, Sunnis and Kurds, the three main ethnic groups in Iraq.
Sunnis, who were promised the crucial defence ministry, claim the majority Shias are blocking their nominee for the post.
