Police say pre-dawn attacks by Islamist militants in the oil-rich city are still underway. The assault comes as Iraqi forces try to drive "Islamic State" (IS) fighters from Mosul, 175 kilometers (109) miles away.
Gunmen wearing suicide vests targeted the main police headquarters as well as checkpoints and patrols on Friday, security officials said.
Witnesses told The Associated Press (AP) that multiple explosions rocked the Kurdish controlled city and that gunfire could be heard throughout the city.
Residents also reported seeing dozens of armed jihadists in the streets several hours into the attack.
In an online statement, IS claimed responsibility for several attacks in and around the northern Iraqi city on Friday.
A Kurdish intelligence officer confirmed that four suicide bombers had attacked the main police headquarters in the city at around 3:00 am (00:00 UTC Thursday).
"The security forces managed to shoot one of them dead, the other three blew themselves up," he told the Agence France-Presse (AFP) news agency.
A total of six policemen and 12 IS fighters were killed during the attacks, AFP said, citing a police official.
Several other targets in the south of the city were also attacked, the officer said, adding that a curfew had been declared.
Power station targeted
Around the same time, suicide bombers stormed the site of a power plant being built by an Iranian company in Dibis, just 55 kilometers (34 miles) away from Kirkuk.
The mayor of Dibis Abdullah Nureddin al-Salehi said at least 16 people were killed in that attack.
"Three suicide bombers attacked the power plant at around 6:00 am (03:00 UTC), killing 12 Iraqi administrators and engineers and four Iranian technicians," al-Salehi told AFP.
The assaults came as the Iraqi government and Kurdish forces are making a major push to drive IS militants from Iraq's second-largest city of Mosul.
There was no immediate claim for the attacks but nearly all similar raids have been carried out by IS.
Kirkuk lies some 290 kilometers (180 miles) north of Baghdad and is claimed by both Iraq's central government and the country's Kurdish region.
mm/rg (AP, AFP)