Influential Iraqi Shiite cleric Muqtada al-Sadr said on Saturday that his followers will resume resistance if US forces do not leave Iraq as scheduled, DPA reported.
"If the Americans don't leave Iraq, we will increase the military resistance and restart the activities of the Mahdi Army," al-Sadr said in a statement, read by a spokesman to thousands of followers in Baghdad.
Tens of thousands, mostly Sadrists, gathered in an anti-US rally in Baghdad Saturday, marking the eighth anniversary of the fall of Baghdad.
Demonstrators gathered in al-Mustansirya Square, eastern Baghdad, carried Iraqi flags and shouting slogans against the US presence in the country. One huge banner read "Get out, occupiers."
Iraqi police and army forces closed roads leading to the demonstration, as thousands of Sadrists walked towards the square.
Al-Sadr, known for his strict anti-US stance, has been a thorn in the side of US forces since the US-led 2003 invasion.
Al-Sadr's statement comes after US Secretary of Defence Robert Gates said that some US troops could stay in Iraq for years, beyond the scheduled withdrawal of all United States forces at the end of 2011.
Gates was speaking to US soldiers in Mosul, around 400 kilometres north of Baghdad, in what is expected to be his last trip to the country, as Gates has said he intends to leave his post later this year.
There are just under 50,000 US soldiers still in Iraq. All troops were supposed to leave the country under a deal negotiated in 2008 by the administration of President George W Bush.
In June 2003, al-Sadr established a military wing, the Mahdi Army, which was involved in major clashes with US forces in Najaf.
Al-Sadr, who is currently living in Iran, is able to mobilize tens of thousands in support of his political aims, although the Mahdi Army has observed a ceasefire since August 2007, which is widely attributed to being behind the decrease of sectarian tensions and violence in Iraq.