Egypt awaited voters' verdict Wednesday on last year's overthrow of Islamist president Mohammed Morsi after polls closed in a two-day referendum on a new constitution.

Officials have said that army chief Abdel Fattah al-Sisi, who toppled Morsi in July, will closely monitor turnout in the vote as he mulls standing for the presidency himself in an election promised for later this year.

U.S. State Secretary John Kerry said he hopes Egypt's constitutional referendum, in which voting ends Wednesday, will be a "transparent and accountable" process.

If there is strong support for the new charter, that may launch a presidential bid by army chief General Abdel Fattah al-Sisi, who overthrew Islamist president Mohammed Morsi in July.

Three members of a trade union abducted last week in Egypt's restive Sinai were freed Wednesday after a ransom was paid for their release, security officials said, with a fourth man still held.

The four men, an official from the labor ministry and the three union members, were snatched while traveling to a conference in the southern resort town of Sharm El-Sheikh.

Voting in Egypt's constitutional referendum was largely incident-free Wednesday after clashes left nine people dead the previous day, with turnout seen as key to a likely presidential bid by the army chief.

Dozens of supporters of ousted Islamist President Mohammed Morsi blocked a metro railway station in a Cairo suburb on Wednesday, security officials said, but there were no reports of disruptions to polling.

Egyptians queued outside polling stations on Tuesday to vote on a new constitution many said they had not read but would approve anyway in support of the army's ouster of Islamist president Mohammed Morsi.

The referendum has been billed by authorities as the first in a series of polls that will restore elected government by the end of the year.

The ruler of Dubai has said he hopes Egyptian army chief Abdel Fattah al-Sisi will not run for Egypt's presidency, days after the general indicated his willingness to stand in the election.

"I hope he stays in the army. And someone else [stands] for the presidency," Sheikh Mohammed bin Rashed al-Maktoum said in an interview with the BBC, excerpts of which were provided Monday by the channel.

A crackdown by Egypt's authorities on the Muslim Brotherhood has left hundreds of Islamic charities struggling for funds, with experts warning the move could impact this week's referendum on a new constitution.

The Tuesday-Wednesday vote has been billed as the first step in Egypt's democratic transition after the military overthrew Islamist president Mohammed Morsi in July following huge protests against his one-year rule.

Police fired tear gas at university students in Cairo on Sunday as they demonstrated in support of ousted president Mohammed Morsi, two days ahead of a referendum on a new constitution.

Clashes between police and the protesters erupted in front of Cairo University, al-Azhar University and Ain Shams University, security officials said, adding that 19 students, including four women, were arrested.

Egypt's army chief Abdel Fattah al-Sisi stepped closer on Saturday to confirming a widely anticipated presidential bid, saying he would stand in elections if there was a popular demand, state media reported.

Sisi is easily the most popular leader in Egypt after toppling Islamist president Mohammed Morsi in July, and military officers have told Agence France Presse he had support within the army to stand in the election due to take place later this year.

Egypt's powerful army chief General Abdel Fattah al-Sisi Saturday urged Egyptians to participate "in force" in next week's referendum on a new constitution, billed as the first stage in the nation's democratic transition.

On July 3, Sisi overthrew Mohamed Morsi after massive protests against the Islamist president's turbulent one-year rule. His ouster triggered unrest that has deeply polarized the country.