Iraqi cleric and militia leader Moqtada al Sadr has threatened to take up arms against American troops again - unless they leave the country by the end of the year.
In a rare interview, he told the BBC that he believed the American forces would not stick to an agreement to withdraw from Iraq by January 2012.
The United States still has about 46,000 troops in Iraq.
Thousands of al Sadr's supporters rallied in Baghdad on Thursday, calling for the Americans to withdraw.
The BBC reports they marched marched in military formation in Sadr City, trampling the flags of the US, Britain and Israel.
Gone were the trademark black shirts of the past. Instead, they wore the colours of the Iraqi flag - red, white and black - with 'Allahu Akbar' (God is Great) emblazoned across the middle.
No weapons were in evidence but al-Sadr made it clear to the BBC that the Mehdi Army was still a fighting force to be reckoned with.
Until January, al-Sadr had spent most of the past two years in self-imposed exile in Iran. But his party did better than expected in last year's election and now he is back.
Al-Sadr controls 39 seats in the 325-member parliament.