By Andrew Quinn and Sattar Rahim

BAGHDAD (Reuters) - Supporters of Shi'ite cleric Moqtada al-Sadr paraded through a Baghdad slum on Friday, burning American flags and shouting anti-U.S. slogans during a visit to Iraq by U.S. Vice President Joe Biden.

Biden was on a trip to Iraq to promote reconciliation between Iraq's fractious groups after six years of bloodshed. He met for breakfast on Friday with his son Beau Biden, who is serving there with the U.S. military.

Biden started his visit to Iraq on Thursday night, after U.S. forces pulled out of Iraq's towns and cities this week under the terms of a bilateral security pact that paves the way for a full U.S. withdrawal by 2012.

After Friday prayers, hundreds and possibly thousands of residents of Sadr City chanted "down, down USA" and burned U.S. flags in protest at Biden's visit. A smaller demonstration also took place in Kerbala, in the Shi'ite south.

Biden helped author a 2006 plan to split Iraq into self-ruled Sunni, Shi'ite and Kurdish enclaves. That plan angered many Iraqis, and was quietly shelved as violence ebbed.

"Biden has come here to divide Iraq according to his plan," said a message from Sadr read out by one Imam in a mosque.

One of Baghdad's blinding dust storms blanketed the city on Friday, grounding helicopters and forcing Biden to reschedule meetings with Iraqi officials.

White House officials said Biden would meet President Jalal Talabani and Prime Minister Nuri al-Maliki, and U.S. commanders and troops, marking the U.S. July 4 Independence Day holiday.

This was Biden's second trip to Iraq this year and his first as vice president. President Barack Obama visited Iraq in April.

MIXED FEELINGS

Biden's visit comes at a critical time in U.S.-Iraqi relations. The bilateral security pact sets a target of pulling all U.S. forces out of Iraq by 2012, putting pressure on Sunni, Shi'ite and Kurdish leaders to resolve disputes over power, oil and land that have stalled reconciliation.

Iraqi politicians had mixed feelings about Biden's visit.

"This visit is important ... the issue of dividing Iraq was rejected by Iraqis, so we don't fear it," said Abdul-Kareem al-Samarrai, head of parliament's Accordance Front Sunni bloc.

"But the reconciliation issue ... should be activated by Iraqis themselves not by others' recommendations."

Obama this week asked Biden to play a coordinating role in the White House on Iraq policy.

The sectarian warfare and insurgency unleashed by the 2003 U.S.-led invasion have receded dramatically over the past year, but there continue to be largescale attacks. Some Iraqis blame U.S. policies for sowing those divisions.

"So far, we haven't experienced neutrality from the U.S. toward the differing parties. They usually back a party against another," said Usama al-Nujaifi, a lawmaker from former Prime Minister Iyad Allawi's Iraqiya list.

Nasseer al-Issawi, from Sadr's movement, called Biden's visit a "public interference in Iraq's affairs."

(Additional reporting by Khalid al-Ansary; Writing by Tim Cocks and Andrew Quinn; Editing by Michael Christie)