That could be good news for Egypt's nascent democracy and its battered economy if it encourages political rivals to fight their battles at the ballot box rather than in the streets, where clashes have alarmed both investors and tourists.

Yet Mursi will have little respite as he braces for the unenviable task of winning public support for urgent economic policies to rein in a crushing budget deficit that are bound to be unpopular in a nation deeply divided by his actions.

"The results of the referendum do not put an end to this state of polarisation in Egyptian politics," said Mustapha Kamal Al-Sayyid, a professor of political science at Cairo University.

The Muslim Brotherhood's party, which propelled Mursi to power in a June election, said its unofficial tally for Saturday's first-round vote on a controversial constitution showed 57 percent of voters backed it, supporting liberal opposition arguments that many felt the document too partisan.

A second round of the vote now looks likely to go the same way on Saturday, as it will be held in districts with more Islamist sympathisers. But even if another, possibly bigger Yes vote pushes the constitution through, the poll on a national charter has underlined the political divisions that Egypt faces.

"This puts major hurdles before President Mursi because the economic measures he planned to introduce ... require a national consensus," said Sayyid, arguing that the Islamists, who have dominated other ballots this year and last, were losing ground.

Mursi's first attempt to implement tax increases, about a week before the vote, lasted only a few hours before he withdrew them amid howls of public anger, which his opponents exploited.

His prime minister has promised a "national dialogue" to explain the government's determination to protect the poor, but Mursi cannot wait long as the measures are seen as essential to securing a vital $4.8-billion International Monetary Fund loan; it was delayed by a month immediately after his policy U-turn.