Shiite radical leader Moqtada al-Sadr warned on Saturday that his supporters will resume armed opposition if US forces stay in Iraq beyond their scheduled departure late this year. "If the Americans don't leave Iraq on time, we will increase the resistance and restart the activities of the Mahdi Army," Sadr said in a fiery statement read by a spokesman to tens of thousands of followers in Mustansariyah Square in northeast Baghdad. He was referring to his militia which mounted repeated uprisings against US-led forces in Iraq before he stood it down in August 2008. "Out, out America," spokesman Salah al-Obeidi repeatedly warned, speaking on the eighth anniversary of the day when Iraqi dictator Saddam Hussein was ousted and Baghdad fell to US-led forces. The Americans must leave, "now, now, now", he warned, reading the statement from Sadr, who divides his time between the central Shiite shrine city of Najaf and neighbouring Iran. "Out, out America," he repeated, at a rally where supporters -- nearly all men -- chanted "Yes, Yes, Moqtada," and set ablaze the US flag and an effigy of former president George W. Bush, who ordered the US-led invasion in 2003. The message from Sadr came a day after US Defence Secretary Robert Gates ended a two-day visit to Iraq, during which he said American forces could stay on beyond 2011 in some numbers, if asked. Nearly 50,000 American troops are still in Iraq Gates asked Iraqi politicians to make that request quickly if they want some American troops to remain. "My basic message to them is (for us to) just be present in some areas where they still need help. We are open to that possibility," the Pentagon chief said. "But they have to ask, and time is running out in Washington," he said, after meeting Prime Minister Nuri al-Maliki, President Jalal Talabani and Massud Barzani, president of the autonomous Kurdistan region in the north. But Sadr on Saturday warned that US forces must leave "to the last soldier and base." Nearly 50,000 American troops are still in Iraq, down from a peak of more than 170,000 after the invasion eight years ago, and ahead of the planned full withdrawal at the end of this year. "Peaceful resistance will end and military resistance in every way will resume," if American forces remain beyond the departure date, said Kamal al-Saadi, a Sadrist lawmaker at the protest. "The voices calling for the occupation forces to stay are weak, while the loud voices are from all Iraqis, who reject the occupiers," he told AFP. Violence in Iraq has plunged since its peak in 2006 and 2007, but security remains a major problem, with bombings, kidnappings and gun attacks happening nearly every day. Many Iraqis worry that security could deteriorate once the Americans leave.US forces are largely engaged in "advise and assist" roles with Iraqi security forces since formally ending combat operations last August, when most troops left the country. General Babak Zebari, the Iraqi armed forces chief of staff, has called the US withdrawal premature, saying his forces would not be able to ensure full security before 2020. The Sadrists are an important component of Shiite Maliki's unity government, with seven ministers in the cabinet and 40 lawmakers in the 325-member parliament. Sadr, who is said to be in his 30s, gained wide popularity among Iraqi Shiites in the months after the invasion and in 2004 when his Mahdi Army militia battled US troops in two bloody conflicts. He was identified by the Pentagon in 2006 as the greatest threat to stability in Iraq. His militia became the most active and feared armed Shiite group, and was blamed by Washington for the death-squad killings of thousands of Sunnis. But in August 2008, Sadr suspended the activities of Mahdi Army, which once numbered in the tens of thousands, following major US and Iraqi assaults on its strongholds in Baghdad and southern Iraq in the spring. After the ceasefire, US military commanders called Sadr's action instrumental in helping bring about a significant decrease in the levels of violence across Iraq. The fiercely anti-American cleric, who has been pursuing religious studies in the Iranian holy city of Qom, is the son of revered Grand Ayatollah Mohammed Sadiq al-Sadr.