Voting is under way for the second and final round of Egypt's referendum on a new constitution.
According to early reports, voters were queuing in large numbers at polling stations.
Opponents of the new constitution say the document favors Islamist parties and ignores the rights of Christians, other minorities, and women.
President Muhammad Morsi says passing the constitution is essential if Egypt is to move toward a more democratic system.
"We should have dialogue and speak kindly to each other," Muhammad Badie, head of the Muslim Brotherhood movement, told Reuters after casting his ballot. "There can be no dialogue by throwing stones, or firing bullets, or using guns. We should have dialogue by speaking nicely to each other. 'When you argue with them, speak to them in a kind way.' Dear people, all of the prophets passed through our beloved country."
Opposition leader Hamdeen Sabahi, who contested the 2012 presidential race and finished in third place, said the new constitution has been written to work perfectly for Islamists.
"Today we are monitoring the voting process. We will react according to the incidents, as we did after the first round anyway," he said. "The result will represent the legal side, but the political and social side is against this constitution, which does not represent a unanimous agreement by the people."
Protests by supporters and opponents of the new constitution turned violent in Alexandria on December 21 when the two groups threw stones at each other.
Police were called in and fired tear gas to disperse the protesters. Sixty-two people, including 12 police officers, were reported injured in the violence.
Unofficial first-round results showed some 57 percent of voters cast their ballots in favor of the new constitution on December 15.
Voting is taking place on December 22 in 17 electoral districts, many of which are considered conservative. However, the close margin and the low turnout in the first round have encouraged those who oppose the document.
If the constitution is adopted, legislative elections will be held within 60 days.
The upper chamber of Egypt's parliament, the Shura Council, will have authority to pass legislation until a new parliament is convened.
On December 21, Morsi made 90 last-minute appointments to the Islamist-dominated Shura Council. If he had waited until after the constitution was approved, he would have had the authority to name just 10 members.
Although Morsi appointed some non-Islamists, the Islamist majority in the chamber remains strong. Opponents accused him of setting up a "token" opposition.
If the constitution fails, an election will be held within 90 days to choose a new constitutional assembly to draft a new version.
Egypt has been divided between Islamists and secularists since the ouster of longtime leader Hosni Mubarak in February 2011.
With reporting by Reuters, dpa, and AP