At least 12 Iraqis were killed and nearly 50 wounded in clashes on Wednesday between police and insurgents in the northern city of Mosul, police said, after two people died in a roadside bombing.
Fighting erupted at around midday (0800 GMT) south-west of Mosul on the west bank of River Tigris, amid sound of loud explosions and heavy gunfire, an AFP correspondent said.
"The hospital received 12 bodies, including two women, and 26 injured, most of them civilians," a doctor at Mosul's Medical City hospital said.
The general hospital said 12 patients had been admitted with injuries, and the Salam Hospital said eight people had been brought in for treatment.
Two insurgents were killed and several others wounded when they attacked the Nabi Girgis district police station with rocket-propelled grenades and guns, Governorate Spokesman Hazem Galawi told a news conference.
Armed assailants also tried to force their way into a branch of the Al-Jazeera bank in the New Mosul district, but security guards fought them off, wounding some insurgents, Galawi added.
US troops had fanned out across various city districts, near the main multinational force base, taking up positions in schools, deserted during the long summer vacations.
At least five bridges were cut off as plumes of smoke rose from the area and masked gunmen took up positions in streets across the south-west of the city, leaving police and national guardsmen deployed on the bridges.
Streets of this city of about 1.75 million people emptied after authorities imposed a curfew from 3:00 pm (1100 GMT) until Thursday morning.
Gunmen forced shopkeepers to close in the Bab al-Toub, Yarmouk, New Mosul and Hospital neighbourhoods of the city centre, the police said.
"Another group of gunmen was also going around distributing weapons to people and urging them to fight the Americans," Police Officer Wael Ismail Ahmed said.
"We received a letter from the insurgents asking us to back off and not to interfere in their fight with the Americans."
Earlier, a man and a woman were killed and two other people received injuries when a roadside bomb exploded in the path of a US military convoy, police said.
Mosul's provincial council recently elected Duraid Kachmula as governor to replace his cousin Ussama Kachmula, who was killed in mid-July along with two bodyguards as he travelled to Baghdad.
Meanwhile, the governor of Iraq's restive Al-Anbar province resigned on Wednesday after three of his sons were released by their kidnappers on the condition he quit.
Abdel Karim Berges told AFP he resigned after his sons, aged 15 to 30, were freed in Fallujah following one week in captivity. He paid no ransom but bowed to the kidnappers' demand that he leaves office, the governor said.
Ramadi's chief administrator Mohammed Abed Awad has taken over his duties.
The sons were kidnapped by gunmen who barged into and torched the governor's family home in Ramadi while he was at work.
US forces arrested an Iraqi police colonel overnight and searched a police station in Ramadi, west of Baghdad, police sources said on Wednesday.
"An American unit went to the home of Colonel Imad Suleiman Falah at 2:30 am (2230 GMT) and arrested him without explanation," policeman Ziya Hadi said.
US soldiers also searched the al-Faruq Police Station in the centre of Ramadi for more than three hours, Hadi said.
A US military spokesman in Baghdad had no information about the reports.