Egyptians must choose between a Muslim Brother or an ex-military man in a presidential run-off, first-round results have indicated.
Up to 50 million Egyptians went to the polls this week in free elections for the first time in their history.
With most votes counted, the Muslim Brotherhood said its candidate Mohamed Mursi had topped this week's poll.
They said he would compete in the second round with former air force chief Ahmed Shafiq, who served as Hosni Mubarak's last prime minister.
The election marks a crucial step in a messy and often bloody transition to democracy, overseen by a military council that has pledged to hand power to a new president by 1 July.
More turbulence could follow if Mr Shafiq is elected, as his foes have already vowed to take to the streets if that happens.
A victory for Mr Mursi could worsen tensions between resurgent Islamists and the army, self-appointed guardian of the state.
Official results are due next week, but a senior judge involved in supervising the vote said Mr Mursi and Mr Shafiq were in the lead, based on figures from 90% of polling stations.
Leftist Hamdeen Sabahy lay third, he said.
The election has split Egyptians between those who oppose an effective return to the Mubarak era and those who fear an Islamist monopoly of ruling institutions.
The run-off will be held on 16 and 17 June.
Former leader Hosni Mubarak, 84, is on trial for ordering the killing of protesters and for corruption. A verdict is due on 2 June.