CAIRO - Egypt's President Mohamed Morsi called a Dec. 15 referendum on a draft constitution on Saturday as at least 200,000 Islamists demonstrated in Cairo to back him after opposition fury over his newly expanded powers.Speaking after receiving the final draft of the constitution from the Islamist-dominated assembly, Morsi urged a national dialogue as the country nears the end of the transition from Hosni Mubarak's rule.Tens of thousands of Egyptians had protested against Morsi on Friday. "The people want to bring down the regime," they chanted in Cairo's Tahrir Square, echoing the trademark slogan of the revolts against Hosni Mubarak and Arab leaders elsewhere.Rival demonstrators threw stones after dark in the northern city of Alexandria and a town in the Nile Delta. Similar clashes erupted again briefly in Alexandria on Saturday, state TV said.Egyptian opposition figure Mohamed ElBaradei said on Saturday the "struggle will continue" after Morsi called for the referendum on a draft constitution."(Morsi) put to referendum a draft constitution that undermines basic freedoms and violates universal values. The struggle will continue," ElBaradei said on his Twitter feed."I renew my call for opening a serious national dialogue over the concerns of the nation, with all honesty and impartiality, to end the transitional period as soon as possible, in a way that guarantees the newly-born democracy," Morsi said.Morsi plunged Egypt into a new crisis last week when he gave himself extensive powers and put his decisions beyond judicial challenge, saying this was a temporary measure to speed Egypt's democratic transition until the new constitution is in place.His assertion of authority in a decree issued on November 22, a day after he won world praise for brokering a Gaza truce between Israel and the Palestinian Islamist Hamas movement, dismayed his opponents and widened divisions among Egypt's 83 million people.Two people have been killed and hundreds wounded in protests by disparate opposition forces drawn together and re-energized by a decree they see as a dictatorial power grab.A demonstration in Cairo to back the president swelled through the afternoon, peaking in the early evening at at least 200,000, said Reuters witnesses, basing their estimates on previous rallies in the capital. The authorities declined to give an estimate for the crowd size."The people want the implementation of God's law," chanted flag-waving demonstrators, many of them bussed in from the countryside, who choked streets leading to Cairo University, where Morsi's Muslim Brotherhood had called the protest.