Egypt have begun voting on constitutional changes drawn up in the weeks since President Hosni Mubarak was overthrown.
The changes, if approved, will reduce the presidential term of office and oblige the leader to appoint a deputy.
The two established political blocs, the National Democratic Party and Muslim Brotherhood, back the proposals, the BBC reports.
But pro-democracy activists say the changes do not go far enough and want the plan rejected. They say the constitution needs to be entirely rewritten before elections can be held.
Crowds of activists once again crammed Cairo's Tahrir Square on Friday, this time to protest against the referendum.
Nobel Peace Prize winner Mohamed ElBaradei, who emerged as an opposition figurehead during the uprising, says it deals only with "minutiae".
"It doesn't talk about the imperial power of the president, it doesn't talk about the distortion of the parliament, it doesn't talk about the need to have an independent constituent assembly that represents everybody," he says in comments carried by AFP.
"So we are going to say no tomorrow. Most of the people who triggered the revolution are going to say no."
The scholars who drafted the changes did not radically overhaul presidential powers because they said the next parliament should entirely rewrite the constitution.