Tens of thousands of Shi'ite youths covered in white shrouds gathered Friday in Iraq's capital for a pro-Hizbullah rally as senior US generals warned that spiraling sectarian violence in Baghdad could lead to civil war. A car bomb killed three policemen in the northern city of Mosul Friday and clashes between Iraqi security forces and Sunni insurgents left one policeman dead and eight people injured, also in Mosul, officials said. Four Shi'ites were shot dead overnight by unidentified gunmen near Baghdad. The Shi'ite youths swarmed the streets of the Shi'ite dominated Sadr City slum, dressed in white shrouds - a symbol of their willingness to die - where the rally was to be held. Waving Hizbullah's yellow flags, they chanted "Death to Israel," "Death to America." Organizers said about 250,000 people had gathered but the estimate was impossible to confirm. The rally, scheduled after Friday afternoon prayers to show support for the Lebanese Shi'ite Hizbullah group in its fight against Israel, was called by radical anti-US cleric Muqtada al-Sadr. But it is not clear if al-Sadr, who lives in the southern holy city of Najaf, will attend. Iraqi government television said the Defense Ministry had approved the demonstration, a sign of the public anger over Israel's offensive in Lebanon and of al-Sadr's stature as a major player in Iraqi politics. Although the rally was not about Iraq, the presence of so many young Shi'ites - most of them from al-Sadr's Mahdi Army militia - adds to tensions in the city that has seen almost daily clashes between Shiite and Sunni extremists. The area around the rally site was sealed off by Mahdi Army militiamen and Iraqi police, who conducted body searches of all those arriving. Cars were banned from entering Sadr City to prevent car bombings. Iraqi police said one al-Sadr follower, on his way to Baghdad, was killed by US troops after he brandished a weapon. However, American officials said two people were shot by US troops, describing them as terrorists. US officials have blamed Mahdi Army for much of Iraq's sectarian violence, which escalated after the February 22 bombing of a Shi'ite shrine in Samarra unleashed a wave of reprisal attacks on Sunnis nationwide.