FAYOUM - Possible presidential candidate Mohamed ElBaradei called on supporters attending his biggest rally so far to campaign for a change in Egypt's constitution. Up to 3,500 Egyptians rallied in Fayoum, south of Cairo, on Friday to support the former UN nuclear watchdog chief. ElBaradei has said he may run in the 2011 presidential vote if there are constitutional reforms, but the existing rules make it almost impossible for an independent to get on the ballot paper. The government insists the system is free and fair. "This is a people's movement and it can only be successful by the will of a people' willing to give their voices to the petition for change and follow it through," ElBaradei told the crowd, adding he planned more campaign visits across Egypt. Organised by a group on the social networking site Facebook, which has a quarter of a million members backing him for the presidency, the rally was the third and biggest of such gatherings so far and drew families from Cairo and Alexandria, as well as Fayoum. Previous rallies have attracted up to 1,500 people. "Who said Egypt has no alternative, ElBaradei is the obvious proof," chanted supporters as ElBaradei emerged from the Mubarak mosque in Fayoum after midday prayers. "This event proves the youths and all Egyptians back the change and reform ElBaradei is calling for," Abdel Rahman Yussef, head of the Facebook group organising the rally, told Reuters. ElBaradei then held the open-air rally in defiance of an emergency law that bans political gatherings. ElBaradei's campaign aims to collect signatures to demand constitutional reforms and lift the emergency law, in place since 1981 and renewed in May until 2011. Mohamed el-Katatni, the leader of the bloc of the Muslim Brotherhood - Egypt's biggest opposition group - in the lower house of parliament, said this week the group would back ElBaradei's campaign for change. Analysts say a petition may not lead to changing the constitution by the 2011 race but could attract international scrutiny of Egypt's sclerotic political system. "Any succession that does not take place within a fully democratic system where elections are fair and the people vote freely has no credibility," ElBaradei told Reuters. Many Egyptians assume Mubarak's politician son Gamal would be a top contender to succeed his father.