Insurgents detonated bombs around a police building in the Iraqi city of Ramadi before storming it wearing explosive-belts, police say.
The number of casualties is unclear, but one report said six people had died and 14 were wounded.
There are reports of hostages as security forces surround the building in the centre of the city.
Ramadi - 100km (60 miles) west of Baghdad - was once a stronghold for the insurgency led by al-Qaeda.
Militant attacks are now intermittent.
AFP news agency quoted two police officers, speaking on condition of anonymity, as saying that two car bombs went off near the Dawlah Kabir Mosque in central Ramadi at around 11:30 (08:30 GMT).
A third car bomb also went off in the city centre, and a fourth car bomb detonated near the police building a short while later.
Two suicide bombers then blew themselves up inside the building before armed insurgents moved in.
Clashes were continuing into the afternoon, the officers said.
The building holds several al-Qaeda suspects in a jail, a police official told Reuters news agency reports.
"Police and army are surrounding the building. There is no exchange of fire, but the security forces are studying storming the building and saving the hostages," the official said.
According to Reuters, three policemen, two of the attackers and a civilian died in the assault and 14 others - including seven police and seven civilians - were wounded, according to Mohammed Fathi, the spokesman for Anbar province.
On Saturday, a suicide bomber targeting Shia pilgrims killed 53 people on the outskirts of the southern city of Basra.
The violence comes only weeks after US troops completed their withdrawal from Iraq and amid a political crisis, which has seen tensions rise between the country's Shia majority and Sunni minority.