BAGHDAD (Reuters) - Thousands of people heeded a call from anti-American cleric Moqtada al-Sadr to protest talks between Washington and Baghdad on keeping U.S. troops in Iraq beyond 2008, but turnout on Friday was lower than past marches.

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Explaining the relatively low numbers, spokesmen for Sadr’s movement said the protests were widely spread through the country but security forces prevented marches in some areas.

In one of the largest demonstrations, several thousand people took to the streets in the Baghdad district of Sadr City, a bastion of Sadr’s Mehdi Army militia. They held up pictures of Prime Minister Nuri al-Maliki dressed as Saddam Hussein.

In the Kadhimiya district in northwest Baghdad, hundreds of demonstrators with raised fists marched behind a banner asking the United Nations to “stand with the Iraqi people against this security deal between the government and the occupation”.

The United States, which invaded in 2003 to topple Saddam Hussein, now has 155,000 troops in Iraq.

It is negotiating with Iraq on an agreement aimed at giving a legal basis to U.S. troops after December 31, when their United Nations’ mandate expires. Sadr’s followers see it as a surrender of Iraq’s sovereignty to an occupying force.

Sadr, backed by a militia estimated to number tens of thousands and popular among Iraq’s Shi’ite poor, has called for protests to continue until the government agrees to hold a referendum on the U.S. presence.

Abdul Aziz al-Hakim, head of the Supreme Islamic Iraqi Council, the biggest Shi’ite group in Maliki’s government, also criticized the planned agreement on a troop extension.

In a statement on his website, he said there was a “national consensus to reject many points raised by the American side as they infringe national sovereignty.”

Sadr pulled his bloc out of Maliki’s government last year in protest at his refusal to negotiate a timetable for a U.S. troop withdrawal.

He called for a million-strong march against the U.S. presence in April but later called it off for security reasons.

On Friday, about 1,200 people marched from the Grand Mosque in Kut, 150 km (95 miles) southeast of the capital, to the Sadr office.

Saad al-Maliki, head of the Sadr office in Kut, said: “As long as there is life, these demonstrations will continue until this agreement is cancelled.”

In the Shi’ite holy city of Najaf, 160 km (100 miles) south of Baghdad, several hundred demonstrators marched, chanting: “Out, out occupier” and “Iraq won’t be an American colony”.

Protests were also held in Basra and Nassiriya. Khalid al-Isawi, head of the Sadr office in Nassiriya, said a similar demonstration would be held every Friday.