Baghdad, Jan 24 (AFP) Thousands of supporters of Iraqi cleric Moqtada Sadr amassed in Baghdad on Friday demanding the ouster of US troops, prompting rival protests seeking to keep the spotlight on months-long calls for reform.

Sadr's march rattled the separate protest movement that has gripped the capital and Shiite-majority south since October, urging a government overhaul, early elections and more accountability.

Thousands of men, women and children massed under grey skies in east Baghdad's Jadiriyah district, chanting "Get out, get out, occupier!"

Some waved signs in Arabic and English reading "Death to America" and one protester carried a cardboard cut-out of US President Donald Trump on the gallows.

Sadr himself did not attend, but a representative read a statement in his name.

It demanded all foreign forces leave Iraq, Iraqi-American security agreements be cancelled, Iraqi airspace be closed to US military aircraft and for Trump not to be "arrogant" when addressing Iraqi officials.

"If all this is implemented, we will deal with it as a non-occupying country -- otherwise it will be considered a country hostile to Iraq," the statement said.

Protesters later began peeling away and had mostly dispersed by early evening.

America's military presence has been a hot-button issue in Iraq since a US drone strike killed Iranian general Qasem Soleimani and a top Iraqi commander in Baghdad on January 3.

Around 5,200 US troops are in Iraq to lead a global coalition fighting the Islamic State group, but Iraq said the strike against Soleimani violated that mandate.

Joint US-Iraqi operations were paused and outraged parliamentarians voted for all foreign forces to leave.

Baghdad offered to discuss a withdrawal timeline with Washington but US special envoy for the anti-IS coalition James Jeffrey said Thursday there was no "real engagement".

Long opposed to US troops, Sadr seized on public anger over the drone strike to plan "a million-strong, peaceful, unified demonstration to condemn the American presence and its violations".

Even his rivals, pro-Iran factions from the Hashed al-Shaabi paramilitary force, agreed to join.

Hashed commander Qais al-Khazali, once mentored by Sadr but now counted among his top competitors, endorsed the rally.

"To Trump, the fool -- the people's message of rejection was clear: if you don't leave voluntarily, you'll be ousted despite yourself," he tweeted.

There had been worries Sadr's supporters might storm the high-security Green Zone, home to the US embassy and other foreign missions, or the main anti-government protest camp in Tahrir Square.

By Friday afternoon, they did not appear to be heading towards either, but a few thousand youths flocked to Tahrir in a bid to keep the focus on their regime change agenda.

"The return of protesters to Tahrir is meant to prove ourselves, first, and to protect its peacefulness," Karrar al-Saadi, a demonstrator in the square, told AFP.

He hinted that with all the attention on Sadr's separate rally, there were fears security forces would try to clear Tahrir Square once and for all.

"When you open up a second front, you leave your back exposed to the enemy -- and by enemy, I mean authorities and anyone seeking to use oppressive tactics," Saadi said.

At least 470 people have died in protest-related violence since October, mostly demonstrators shot by live rounds or military-grade tear gas canisters.

Grand Ayatollah Ali Sistani, Iraq's top Shiite authority, also tried to refocus attention on reform in his weekly Friday sermon.

He admonished authorities for "procrastinating" on reform promises and parties for being "very late" in naming a successor to premier Abdel Abdel Mahdi, who has acted as caretaker since resigning in December.

Sadr, 46, battled US forces with his Mehdi Army militia after the 2003 US-led invasion but is now a fickle politician, notorious for switching alliances quickly.

He backed anti-regime protests early on, but also controls parliament's largest bloc and top ministerial posts.

His spokesman Saleh al-Obeidy hinted that while others blamed either Washington or Tehran for Iraq's instability, Sadr would choose a middle path.

"We believe that both are behind this ruin, and Sadr is trying to balance between the two," he said.

Carnegie Middle East Center expert Harith Hasan said Sadr sought to sustain his "multiple identities".

"On the one hand, (he seeks to) position himself as the leader of a reform movement, as a populist, as anti-establishment," Hasan told AFP.

"On the other hand, he also wants to sustain his image as the leader of the resistance to the 'American occupation'," partly to win favour with Iran.

Playing both sides could empower Sadr as divided parties negotiate over the next premier, but the intra-Shiite alliance on display Friday was not likely to last, Hasan added.

"Once this protest is over, this honeymoon between Sadr and the other pro-Iran factions will eventually vanish," he said. (AFP)

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