Young people traded mobile-phone pictures of ink-stained fingers that showed they voted. Others called relatives to boast of casting the first vote of their lives. In the well-off Cairo neighborhood of Maadi, a man hoisted his elderly, infirm father on his shoulder and carried him to a polling station.
"My vote today will make a difference. It's as simple as that," said first-time voter Hossam Bishay, 48.
The first test of Egypt's transition to democracy offered ominous hints of widening sectarian division, however.
Many were drawn to the polls in a massive, last-minute effort by the Muslim Brotherhood, the Islamist group that is Egypt's largest and most coherent political organization after the widely despised National Democratic Party of former President Hosni Mubarak, who was ousted last month in a national popular uprising.
Among other changes, the constitutional amendments would open elections to independent candidates, allowing parliamentary and presidential elections to replace the caretaker military government by early 2012.
Critics say that would allow the Brotherhood and NDP to easily outpoll the dozens of political groups born out of the anti-Mubarak uprising, dividing power between former regime loyalists and supporters of a fundamentalist state -- a nightmare scenario for both Western powers and many inside Egypt.
Among those most fearful of the Brotherhood's rising power are Egypt's estimated 8 million Coptic Christians, whose leaders rallied the faithful to vote "no."
"If the Brotherhood comes to power, they will not benefit anyone, Muslims or Christians," Fawziya Lamie, a 39-year-old Christian nanny, said after casting her "no" vote in the Cairo district of Manial.
The NDP is blamed for the rampant corruption and the fraud that marred every election during Mubarak's 29-year rule, and its members have been accused of attempting to disrupt Egypt's transition to democracy for fear of losing further power.
Reform campaigner Mohamed ElBaradei and a group of his supporters were pelted with rocks, bottles and cans outside a polling center at Cairo's Mokattam district in an attack he blamed on followers of the old regime.
The day was otherwise almost entirely peaceful. The Egyptian Association for Community Participation Enhancement said it expected the turnout to reach 50 percent, more than three times the average level in the rigged elections under Mubarak.
Hundreds of Egyptians formed lines outside polling centers before they opened. They snaked along the streets in Cairo and other cities, with men and women standing in separate lines as is customary in the conservative and mainly Muslim nation.
"This is a historic day for Egypt," Deputy Prime Minister Yahya al-Gamal said after casting his vote in Cairo. "I had never seen such large numbers of voters in Egypt. Finally, the people of Egypt have come to realize that their vote counts."
Saturday's vote was by far the freest since the military seized power in a 1952 coup, toppling the monarchy and ending decades of a multiparty system that functioned while Britain was Egypt's colonial master. Only men with military backgrounds have ruled Egypt since.
While Mubarak's overthrow has left Egyptians euphoric about their newfound freedoms, many are also worried about the social tensions and instability that could spiral in the wake of the autocratic leader's departure.
Christian-Muslim clashes this month left at least 13 killed and more than 100 wounded in the worst sectarian clashes in years. On Jan. 1, a suicide bomber blew himself up outside a church in the Mediterranean port city of Alexandria, killing at least 22 worshippers and wounding scores. A few days later, a policeman shot dead an elderly Christian man on a train.
The Brotherhood, which has strongly campaigned for the adoption of the changes, advocates the installment of an Islamic government in Egypt. The ambivalence of its position on what role women and minority Christians play under their hoped-for Islamic government -- like whether they could run for president or be judges -- worry large segments of society.
In the province of Luxor, thousands of Brotherhood supporters and Salafis, zealous adherents to practices from Islam's early days, held separate demonstrations in the city center to campaign for a "yes" vote.
Churches handed out fliers to worshippers calling on them to vote "no."
To the north in the province of Assiut, home to one of the country's largest Christian communities, priests organized buses to ferry worshippers from churches to polling centers to cast their "no" vote. Islamists using loudspeakers in pickup trucks roaming Assiut's streets were calling on voters to cast "yes" ballots.
"The voice of freedom, truth and power is the voice of The Muslim Brotherhood," said one bearded Islamist. "No voice is louder than the voice of Islam."
"Marking your ballot with a 'yes' is a religious duty," said another.
"What worries me is that this is going to be a rehearsal for the upcoming parliamentary elections, playing with not only religion but the country's democratic future," said Sameh Fawzy, a Christian commentator. "This is very dangerous."
Hossam Tamam, an expert on religious groups, said the polarization over the amendments has taken sectarian overtones, with the "yes" vote associated with Muslims and the "no" with Christians.
"The Brotherhood's discourse included intimidation, pressure and exploitation of ordinary Egyptians," he said. "In response, we have seen a sectarian polarization with all Christians voting 'no,' emboldening the Islamists to label the 'no' vote a Christian choice."
Voters were asked to choose 'yes' or 'no' for the whole package of nine changes, which would also impose presidential term limits and curtail 30-year-old emergency laws that give police near-unlimited powers.
Preliminary results will be announced Sunday.
The attack on ElBaradei, the former head of the U.N. nuclear watchdog agency, forced him to flee in an SUV without casting his ballot. The crowd also smashed the car windows and shouted, "You traitor. We don't want you." ElBaradei supporters at the scene countered by chanting"we want you."
The Nobel laureate later tweeted that "organized thugs" were to blame for the attack. In a second Twitter posting, he said Mubarak regime figures were seeking to undermine the revolution, a reference to the Jan. 25-Feb. 11 uprising.
More than half of Egypt's 80 million people are eligible voters. The military, in a bid to get the vote out, has decreed that they would be allowed to cast ballots at any polling center in the country with their national ID cards the only required proof of identity. They were required to dip their index finger in ink after voting to prevent multiple balloting.
The constitutional amendments were drawn up by a panel of military-appointed legal scholars and intended to bring just enough change to the current constitution -- which was adopted in 1971 and suspended by the military after it came to power -- to ensure that upcoming presidential and parliamentary elections are free and fair.