Cairo, Egypt (CNN) -- Mohamed ElBaradei, an Egyptian presidential candidate and Nobel laureate, was attacked by thugs at a polling station in Cairo on Saturday, his brother told CNN.

ElBaradei described the attack, which occurred during a referendum on changes to the constitution, on his Twitter account. Voting was completed Saturday evening, when all polling districts were reported closed, according to the judicial committee overseeing the elections.

"Went 2 vote w family attacked by organized thugs," he tweeted. "Car smashed w rocks. Holding referendum in absence of law & order is an irresponsible act."

The former head of the International Atomic Energy Agency also tweeted that two members of his campaign team were detained at a separate polling station in Cairo.

ElBaradei said the two women were serving as official monitors at the polling station when they were detained.

"Disgusting," ElBaradei said in another tweet.

His brother confirmed the attack. Egyptians streamed to the polls Saturday to vote on proposed constitutional amendments, the first democratic initiative after the fall of autocratic president Hosni Mubarak's regime.

"Mr. Mohamed ElBaradei's car was attacked by thugs who threw rocks at it and prevented him from entering the Mokatam voting poll," Ali ElBaradei told CNN. "He did not vote today."

The attackers also chanted slogans against him, the brother said.

A military official told CNN he was not aware of the incident.

"We have been on the ground all day securing the polls along with the police and with the aid of the neighborhood watch groups too," said Maj. Alla al Iraqi of the military's press office. "There have not been any incidents of violence or clashes. Any minor arguments I witnessed between those who were voting yes or no were resolved between one another. Today has been a model for democracy."

An estimated 45 million Egyptians are eligible to vote in what is widely viewed as the country's first free election in decades, and the poll sets the stage for parliamentary and presidential elections later this year.

Many voters were exuberant over the occasion.

"I am very, very happy," said Mohamed El Hourushy, a 19-year-old political science student. "This is something I've been fighting for all my life. I didn't think I would live to see this scene."

The proposed amendments include limiting the president to two four-year terms, capping emergency laws to six months unless they are extended by public referendum and placing elections under judicial oversight.

Opponents say the proposed amendments were rushed and fall short of the people's demands. Many demand a new constitution and claim an early referendum gives an unfair edge to the Muslim Brotherhood and remnants of Mubarak's National Democratic Party -- well entrenched and politically savvy groups that are much better prepared to mobilize voters than newer factions still scrambling to get organized.

But presidential candidate and head of the Arab League Amre Moussa, who is urging a "no" vote, lauded the referendum as "the first official step towards the democracy called for in the January 25 movements."

" 'Yes' or 'no' is not the issue -- that Egyptians are participating and voting today is what's important," he said.

The polls opened at 8 a.m. at more than 13,000 polling stations across the country, and many arrived early to polling stations to beat the crowds.

Outside the polling station at Kasr el Eini el Doubara -- a Cairo language school -- about 50 people lined up just before voting started. Many Egyptians, even senior citizens, said this was their first time voting.

"It feels good," said 80-year-old Nadia Risk. "It feels like, although I'm a senior citizen, I might be able to contribute to something that will be very democratic."

"I feel my vote will make a difference for the first time in my life," said 58-year-old Ibrahim Fahmy.

"I was born in 1952 during the first revolution and ever since I did not feel this country belongs to me. This is the first time I feel my vote will make a difference."

Voters filed into a room where they picked up their ballots and went behind a blue curtain to mark their vote. A check inside the green circle was a "yes" vote. The black circle was a "no" vote.

Election officials marked voters' fingers with pink ink to keep them from voting more than once.

At about 8:30 a.m., Moussa, who called on Egyptians to vote against the amendments, arrived to cast his vote. A throng of reporters and cameramen surrounded him as he made his way to the ballot box.

"I honestly passed by many polling stations and they were all filled with people awaiting to make their decision. It is our duty to accept whatever they decide," Moussa said.

"It's very strange to have a referendum organized and implemented in one month," said Karim Elias, a 33-year-old software engineer who said he voted "no."

"I want a new constitution. I want something that represents the Egyptian people," Elias said.

Judge Mahmoud Atiya -- the head of the judicial committee overseeing the referendum -- told CNN the next step in the transition to a civilian government depends on the outcome of Saturday's referendum.

If the measures are voted down, the military will go back to the drawing board, Atiya said, and eventually issue another order, in accordance with the constitution, on what will come next in the transition to a civilian government.

It's not clear exactly what the army's instructions will be in case of a "no" vote or whether they'll extend their self-imposed six-month deadline to transfer power to an elected civilian government.

If the measures are approved, the military will move forward with parliamentary elections in June, Atiya said.

Analysts say it's hard to predict the outcome of the vote.

The ruling Egyptian Armed Forces announced this week that thousands of international monitors would be in place at polling stations across the country to ensure transparency in the voting process. More than 30,000 soldiers and security agents are also manning polling stations, security officials said.

Journalists Dina Amer and Mohamed Fadel Fahmy contributed to this report