At least 33 people have been killed in car bombs and other attacks in Samarra, north of Baghdad, police say.
Three car bombs were reported in quick succession
Two blasts went off outside the mayor's office. A US convoy thought to be trying to reach the scene was also hit.
There are also reports that militants attacked three police stations, killing and wounding a number of policemen.
Samarra has been cited by the Iraqi government as an example of how they have been able to restore order to areas formerly controlled by rebels.
US and Iraqi forces seized control of the Sunni Muslim city in early October.
The BBC's Claire Marshall in Baghdad says that on the eve of an attack on Falluja, events in Samarra seem to demonstrate that it takes more than a large scale military assault to bring a town fully under control.
The attacks, in which more than 48 people were been injured, appear to have been well co-ordinated.
Three car bombs were reported within quick succession from about 0640 GMT on Saturday morning.
Three hours later, there was another car bomb attack on a police station in the south of the town. Ten people were killed.
"I saw a dead National Guard burning on the ground,"
one witness told Reuters news agency.
"I saw a car trying to reach the town hall. When police stopped it, it exploded," said bookshop owner Mohammed Ahmed.
Insurgents have frequently targeted Iraqi officials in Samarra, whom they regard as collaborators with the US.
A local police chief, Brigadier General Abdul Razak Mohammed
al-Jarmani, was reported to have been killed in the car bomb explosions.
The dead included 24 policemen, three national guards and six
civilians, French news agency AFP reported.
The town, about 95km (60 miles) from Baghdad and with a population of about 200,000, has long been a stronghold of elements opposed to the US-led occupation.
In a separate development, 20 US soldiers were wounded in another restive Sunni city, Ramadi.
A US spokesman said they were wounded during increased security operations in the city, but gave no further details.
Bombs pound Falluja
The attacks happened as US forces appeared poised to launch a major offensive against the insurgent city of Falluja.
A doctor at the city's only hospital told the BBC on Saturday that between 100 and 200 US bombs had hit the city over 24 hours, killing two people and injuring seven on Friday.
Speaking by telephone, Dr Ahmed Ranem accused US-led forces of seeking to "destroy" the city where many people, he added, remained in their houses.
No foreign fighters had been admitted to his hospital as casualties, he said, despite US reports that forces loyal to Abu Musab al-Zarqawi are active in Falluja.
"Can the bombs that fall on Falluja see foreign fighters?" the doctor asked, saying casualties at Falluja General Hospital included women and children.
Renewed US artillery attacks on the city overnight damaged a storehouse for medical supplies as well as dozens of homes, witnesses said.
Overnight a column of armoured vehicles and Humvee jeeps carried out attacks in the outskirts of Falluja.
US Marine commanders described this to reporters as a feint designed to draw out the rebels and provide fresh targets for the air power and artillery.
BBC correspondent Paul Wood, who is embedded with the US Marines near the city, says this is all part of what appears to be a steady increasing of pressure on the insurgents.
He says US commanders are confident of their victory but says many questions still remain, such as whether the insurgents will stay and fight.
UN Secretary General, Kofi Annan, has written to both the US and the Iraqi to ask them to think carefully before they embark on the operation.