Egyptians were being asked on Saturday to vote on constitutional amendments that would allow President Abdel-Fattah el-Sissi to stay in power until 2030.
The three-day voting period for the nationwide referendum starts at 9am (0700 GMT) on Saturday on proposed changes that would also further enshrine the military's role in politics.
Parliament, packed with el-Sissi supporters, overwhelmingly approved the amendments on Tuesday.
The proposed changes are seen by critics as another step back toward authoritarianism.
The referendum comes eight years after a pro-democracy uprising ended autocrat Hosni Mubarak's three-decade rule, and nearly six years after el-Sissi led a popular military overthrow of the country's first freely elected but divisive Islamist president, Mohammed Morsi.
The vote comes as authorities waged an unprecedented crackdown on dissent in recent years, arresting thousands of people mostly Islamists but also prominent secular activists and rolling back freedoms won in 2011.
Two international rights groups, Human Rights Watch and the International Commission of Jurists, on Saturday urged the Egyptian government to withdraw the amendments.
Human Rights Watch said the amendments consolidate authoritarian rule. It argued that a free and fair vote would be impossible in what the group described as a relentless crackdown on fundamental freedoms.
The Civil Democratic Movement, a coalition of liberal and left-leaning parties, has urged people to participate in the referendum by voting "No".
The coalition said it used social media to spread its message, noting that it was banned from hanging banners in the streets to call on voters to reject the amendments.
The amendments extend a president's term in office from four to six years and allow for a maximum of two terms. But they also include an article specific to el-Sissi that extends his current second four-year term to six years and allows him to run for another six-year term in 2024 potentially extending his rule until 2030.
El-Sissi was elected president in 2014, and re-elected last year after all potentially serious challengers were either jailed or pressured to exit the race.
The amendments declare the military the "guardian and protector" of the Egyptian state, democracy and the constitution, while also granting military courts wider jurisdiction in trying civilians.