Egypt's opposition is calling for mass protests on Tuesday after Islamists backing President Mohammed Morsi claimed victory in the first round of a referendum it alleges was riddled with polling violations.

The opposition coalition, the National Salvation Front, urged Egyptians to "take to the streets on Tuesday to defend their freedoms, prevent fraud and reject the draft constitution" ahead of the next round of voting on Saturday.

Islamists backing a new constitution for Egypt claimed victory Sunday in the first leg of a referendum, but the opposition alleged polling violations and called for nationwide protests ahead of next weekend's second leg.

The two sides' positions drew out the deep uncertainty and division seen in Egypt over the past three weeks, a period marked by mass protests and deadly clashes.

A slender majority of Egyptians approved a disputed constitution backed by President Mohammed Morsi and Islamist allies in the first round of a referendum, Islamists and an opposition group said Sunday, citing unofficial results.

Saturday's voting in 10 provinces including Cairo came after weeks of mass protests organised by an opposition coalition that initially aimed to torpedo the referendum, before it instead issued last-minute calls for the draft's rejection.

Egypt's ousted dictator Hosni Mubarak, who is serving a life sentence, was injured when he slipped in a prison shower on Saturday, the official MENA news agency reported.

The former president, who was sentenced over the killings of protesters during the uprising that toppled him last year, was being treated for a head wound and bruising in the medical ward of the south Cairo prison, MENA reported.

Egypt's opposition cried fraud in a referendum which started on Saturday on a new constitution, accusing President Mohamed Morsi's Muslim Brotherhood of vote-rigging to have the Islamist-backed text adopted.

But the National Salvation Front opposition coalition did not immediately make good on a threat to call a boycott if it perceived violations, instead stepping up an appeal to Egypt's 51 million voters to reject the draft charter.

Egyptians began voting on Saturday in a two-round referendum on a new constitution supported by the ruling Islamists but bitterly contested by a secular-leaning opposition that has waged weeks of protests.

President Mohammed Morsi's determined push to see through the draft charter led to street clashes in Cairo last week between the rival camps, with eight people killed and hundreds injured.

Stone-throwing clashes broke out Friday in the Egyptian city of Alexandria between Islamists and opposition protesters, on the eve of a highly charged referendum on a new constitution, witnesses and state media said.

Fifteen people were wounded, according to medics, and several cars set on fire in the violence in the Mediterranean city, Egypt's second largest.

Egypt's mainly secular opposition gathered for last-ditch protests on Friday against a new constitution it slams as divisive, but on the eve of a referendum it held back from urging a boycott.

The opposition National Salvation Front expressed "deep concern" over the vote called by Islamist President Mohammed Morsi, which is intended to complete the transition from veteran strongman Hosni Mubarak's rule.

Spain announced Thursday it had seized 28 million euros ($37 million) in financial products, luxury cars and buildings linked to ousted Egyptian president Hosni Mubarak.

Egypt requested Spain's help under U.N. anti-corruption conventions to find and block assets owned by Mubarak, his family, top aides and their related companies, a national police statement said.

A divided Egypt is being called to vote in a referendum Saturday on a new constitution that the secular opposition fears will be used by President Mohamed Morsi and his Muslim Brotherhood to usher in an Islamist interpretation of laws.

Protests over the draft charter, and over near-absolute powers that Morsi gave himself for two weeks to push it through, have failed to sway the president from his path.