At least five people have been killed and up to 40 injured in clashes in Fallujah after Iraqi troops opened fire on Sunni protesters and worshippers, officials have said.

Friday's incident is the most violent yet since Sunni demonstrators began taking to the streets in late December to challenge Prime Minister Nuri al-Maliki's Shia-led government.

The demonstration in the city, located about 60km west of Baghdad, the capital, was one of several that began after Friday prayers across the country.

Protesters are demonstrating against what they see as the increasing marginalisation of Sunnis since the fall of Saddam Hussein.

Authorities said the clashes were sparked when the army arrested three protesters and clashed with rock-throwing demonstrators trying to block a major highway.

A hospital source told the Reuters news agency: "We received three bodies with gunshot wounds in the abdomen, back and shoulder."

A local television channel showed footage of demonstrators approaching the army vehicles and throwing stones and water bottles while troops tried to keep them away began firing in the air.

It showed at least one soldier aiming his rifle at demonstrators.

An eyewitness confirmed to Reuters that troops had fired in the air to disperse protesters, but later some soldiers fired shots at protesters who approached their military vehicles and set one of them on fire.

Divisions among Shias, Sunnis and ethnic Kurds persist a year after the last US troops left the country.

Source: Agencies