Insurgents have attacked US bases and government offices in Ramadi, in central Iraq, and then dispersed throughout the city, residents say.
Scores of heavily-armed insurgents fired mortars and rockets at the buildings and then occupied several main streets.
The attack came as local leaders and US military officials were meeting at the al-Anbar provincial governor's office.
Ramadi has been a rebel stronghold for many months.
Residents told the Reuters news agency that hundreds of heavily armed men in masks were patrolling the main streets of the city and had set up checkpoints.
Leaflets
Residents said leaflets distributed by the men declared that al-Qaeda in Iraq, the group run by Abu Musab al-Zarqawi, was now in control of the city.
Map showing Ramadi
"Its followers will burn the Americans and will drive them back to their homes by force. Iraq will be a graveyard for the Americans and its allies," one leaflet declared.
Residents said there was no noticeable presence of US or Iraqi forces in the city after the attacks.
It is not known whether there have been any casualties.
The attack came as 2000 US Marines and 500 Iraqi soldiers launched an offensive against insurgents in Hit, east of the River Euphrates, not far from Ramadi.
The US military said the town was "suspected to be an al-Qaeda in Iraq safe area and base of operations for the manufacture of vehicle car bombs."