Thousands of Sunnis demonstrated across Iraq on Friday, in the latest of nearly two weeks of rallies criticising the country's premier and demanding the release of prisoners they say are wrongfully held.

The protests, which come amid a political deadlock between Prime Minister Nuri al-Maliki's Shiite-led government and a secular Sunni-backed party that is in his cabinet but publicly opposes him, have blocked off a key trade route and forced heavy security measures.

The demonstrations have been given a cross-sectarian boost in recent days thanks to public displays of support by powerful Shiite cleric Moqtada al-Sadr who joined in Friday prayers at a major Sunni mosque in Baghdad, symbolically backing rallies against a premier with whom he has often clashed.

Demonstrators gathered at the Abu Hanifa mosque in the mostly-Sunni neighbourhood of Adhamiyah, but were barred by security forces from leaving the compound to rally on the street, a correspondent said.

The protesters held up banners calling for a mass prisoner release, stronger human rights provisions in Iraq's prisons, and a repeal of current anti-terror legislation.

They have called for the release of prisoners they say were detained because of their Sunni background, and an end to the alleged misuse of anti-terror legislation by the Shiite-led authorities against their community.

"Baghdad, free, free! Iran, go away!" they shouted, a reference to their belief that Maliki's government is beholden to Iraq's Shiite neighbour Iran.

"How much longer will our children stay in prisons for no other reason than being Sunni," asked a man who gave his name as Abu Abdullah.

Another protester who identified herself as Umm Mohammed said: "My three children were arrested four years ago for no reason and I ask Maliki -- release them."

Meanwhile Sadr, whose movement counts 40 lawmakers and five ministers among its supporters, attended Friday prayers at the Abdul Qader Gilani mosque, a prominent Sunni mosque in Baghdad.

Large-scale protests also took place in several cities north of Baghdad in Salaheddin, Diyala, Kirkuk, and Nineveh provinces, while demonstrators in western Anbar province continued to block off a highway linking Iraq to Syria and Jordan for a 12th successive day.

"We are demonstrating to support the protesters in Fallujah, Mosul, Samarra and other Iraqi cities," said Ismail al-Hudaidi, a protest organiser in Kirkuk's eponymous provincial capital.

Sheikh Ahmed Samarraie, imam of a mosque in Kirkuk city, added: "We are protesting to simply say: no to injustice, and yes to the release of innocent prisoners."

On Thursday, the justice ministry said 11 female prisoners had been released and 13 others had been transferred to jails in their home provinces, two days after Maliki offered to push for the release of hundreds of female detainees in a bid to fulfil one of the protesters' demands.

The premier on Friday called for protesters and security forces to act with restraint, after earlier this week warning demonstrators that the state could intervene to end rallies.

The protests began on December 23, and were sparked by the arrest of at least nine guards of Finance Minister Rafa al-Essawi, a Sunni Arab and a leading member of the secular Sunni-backed Iraqiya bloc, which is part of Maliki's unity government but frequently criticises him in public.