Polls reopened at 9 a.m. (0700 GMT) Sunday. Voting will continue through Monday to allow maximum turnout, which the government hopes will lend the referendum legitimacy.
Election officials say results are expected within a week.
Opposition parties have called on voters to reject the changes, blasted by critics as a major step back to authoritarianism.
Voting comes amid an unprecedented crackdown on dissent since the 2013 military ouster of an elected but divisive president.
Sisi came to power in 2014 and was re-elected for a second four-year term last year.
Trucks with loudspeakers drove around central Cairo Sunday morning urging high turnout, AP reported.