Twenty-eight female guards were unfairly dismissed by a security company because the client‚ Metrora.
The vehicle detonated in central Kirkuk was painted to appear as though it were a police car, and the militants who sought to seize the compound were dressed as policemen, witnesses said.
The attack shattered a relative calm in recent days in Iraq, which has been grappling with a political crisis pitting Prime Minister Nuri al-Maliki against his former government partners amid weeks of ongoing protests calling for him to resign, less than three months before key provincial elections.
No organisation immediately claimed responsibility for the attack, but Sunni militants, including al-Qaeda's front group in Iraq, frequently target security forces and government targets in a bid to destabilise the country and push it back towards the sectarian bloodshed that blighted it from 2005 to 2008.
The initial suicide car bomb was set off during morning rush hour and was quickly followed by three gunmen, dressed in police uniforms, armed with hand grenades and suicide vests, bursting through the main gate of the Kirkuk police compound in the direction of the headquarters building.
They threw multiple grenades as they sought to reach the building but were killed before they could get there, witnesses said.
"I saw a vehicle stop at the checkpoint at the main entrance, and the police started checking it," said Kosrat Hassan Karim, who was nearby when the attack took place. "Suddenly, a loud explosion happened, it was terrifying. I saw many people killed inside their cars - I have never seen such a big explosion in my life."
Brigadier General Natah Mohammed Sabr, the head of Kirkuk city's emergency services department, put the toll at 30 dead and 70 wounded.
In addition to the casualties, the attack caused massive damage to nearby buildings and shops, according to an AFP journalist at the scene.
Kirkuk, an ethnically mixed city 240km north of Baghdad, lies at the heart of a swathe of disputed territory claimed by both the central government and Iraq's autonomous northern Kurdish region.
Attacks are dramatically lower across Iraq since their peak in 2006 and 2007, but bombings and shootings remain common. - Sapa-AFP