Egypt's largest opposition group has strongly condemned government moves to hold a controversial referendum just days after its approval by parliament.
Opposition MPs branding the amendments a "constitutional putsch"
The Muslim Brotherhood group says the tight schedule is designed to prevent effective campaigning by those opposed to the proposed constitutional changes.
The vote will be held on Monday, according to a presidential decree.
Supporters say the changes strengthen democracy; opponents say they cement the creation of a police state.
President Hosni Mubarak, whose ruling National Democratic Party dominates the 454-member parliament, has proposed changes to 34 articles of the constitution.
Parliament approved the amendments on Monday in a session boycotted by opposition groups and they now go to popular referendum.
The vote had initially been expected to take place on 4 April, commentators say.
While the government and its supporters have hailed the measures as long overdue reforms, Amnesty International says they amount to the biggest erosion of human rights since emergency laws were introduced in 1981.
"The new date of the coming referendum stunned all," said Muhammad Habib, deputy leader of the official banned-but-tolerated Brotherhood.
"It aims at reducing the time available for the opposition to hold popular activities opposing the constitutional amendments."
Political scientist Mustafa Kamal Sayyed told AFP news agency that it was possible to hold the referendum quickly, but the result would lack credibility.
"I think the government would just like the ceremony for the referendum but not the referendum itself or any popular participation," he said.
"We know from past experience that the results announced from all referenda do not correspond to reality," he said.
Past elections have been marred by widespread accusations of government-engineered fraud and intimidation of opponents.
The changes prohibit parties from using religion as a basis for political activity, give the president power to dissolve parliament and end judicial monitoring of election.
BBC Middle East analyst Roger Hardy says reformists are convinced the constitutional changes are the final death blow to what remains of Egyptian democracy.
The Muslim Brotherhood says it hasn't decided whether to call for a boycott of the referendum, or for a "no" vote.