Egyptians have begun voting on constitutional changes that will potentially allow President Abdel Fattah al-Sisi to stay in office until 2030 and bolster the role of Egypt's powerful military.
Mr Sisi's supporters say the changes are necessary to give him more time to complete major development projects and economic reforms.
Critics say they concentrate more powers in Mr Sisi's hands and return Egypt to an authoritarian model.
While the changes are widely expected to pass, observers say the size of the turnout will be watched as a test of Mr Sisi's popularity, which has been dented by economic austerity measures since 2016.
If approved, the amendments would extend Mr Sisi's current term to six years from four and allow him to run again for a third six-year term in 2024.
They would also grant the president control over appointing head judges and the public prosecutor from a pool of candidates.
They would task the military with protecting "the constitution and democracy and the fundamental makeup of the country and its civil nature".
Opponents say the changes are being rushed through without proper public scrutiny. Officials say Egyptians from all walks of life were given a chance to debate the amendments, and that all views were factored into the final proposals.
"They are not giving us time to even organise a campaign to ask people to vote no," said Khaled Dawoud, a member of the opposition Civil Democratic Movement.
"This is the final deathblow after all the ambitions we had after the 2011 revolution," said Mr Dawoud, referring to the uprising that ousted Hosni Mubarak after 30 years.
Mr Sisi came to power after spearheading, as defence minister, the removal of the Muslim Brotherhood's Mohamed Mursi as president in 2013 in the turbulent years after the uprising.
Mr Sisi was elected as president a year later and won a second four-year term last year.
Under Mr Sisi, Egypt has witnessed a crackdown on dissent that rights groups say is unprecedented in its recent history.
Media and social media are tightly controlled.
Egypt's 596-member parliament, dominated by Mr Sisi's supporters, approved the amendments on Tuesday, voting by 531 to 22 in favour.
Some 55 million of Egypt's nearly 100 million population are eligible to vote in the referendum, which will be held over three days. The result is expected in the days after Monday's final day of voting.