A crowd of people blocked Mohamed ElBaradei from entering a polling station in Cairo on Saturday to cast a vote in Egypt's constitutional referendum, shoving him and smashing his car window with rocks as he left.
"We don't want you, we don't want you," a crowd of youths chanted at ElBaradei, who has said he wants to run for president. He was shoved after joining a queue of people seeking to vote.
"We don't want an American agent," said another.
ElBaradei, a contender for Egypt's presidency, later slammed as "irresponsible" the holding of a referendum without law and order after he was stoned by Islamists at a polling station.
"He lives in the United States and wants to rule us. It's out of the question," one of them said.
ElBaradei beat a retreat to his car and left without voting at the polling station in Muqattam, a largely poor district in south Cairo.
"Went 2 vote w family attacked by organized thugs. Car smashed w rocks. Holding referendum in absence of law & order is an irresponsible act," he wrote on Twitter.
ElBaradei's brother Ali said he was unhurt. Muqattam is close to the scene of recent deadly clashes between Egypt's Christian Copt community and Muslims.
Members of the crowd interviewed by AFP before the assault identified themselves as Islamists without elaborating on their precise allegiance.
An official from the Muslim Brotherhood, the largest and most organized opposition movement, denied members of his group were involved.
"There are many types of Islamists. It's impossible that it was Muslim Brotherhood members, 100 percent impossible. We work with ElBaradei and coordinate with him," Gamal Nassar told AFP.
An estimated 45 million Egyptians were being asked to say "yes" or "no" to a package of constitutional changes intended to guide the Arab world's most populous state through fresh presidential and parliamentary polls within six months.
ElBaradei, a staunch opponent of the planned changes, is widely respected on the world stage for his work as an international diplomat.
But under the Mubarak regime, he came under repeated criticism for his frequent trips abroad and was accused of being out of touch with the reality of Egyptian life.
He went to vote in Saturday's referendum on constitutional reform after returning from a speaking engagement in New Delhi sponsored by India Today.
The main advocates of a "yes" vote have been the Muslim Brotherhood -- powerful and well-organized despite being outlawed under Mubarak -- and elements of the former ruling National Democratic Party.
Critics say they are the ones who stand to benefit if elections are held too quickly, without giving time for groups stymied by three decades of authoritarian rule to organize at grass roots level.