Thousands of opponents and supporters of Egypt's Islamist president staged rival rallies in the nation's capital Tuesday, four days ahead a nationwide referendum on a contentious draft constitution.

The demonstrations got underway just hours after masked assailants set upon opposition protesters staging a sit-in at Tahrir Square, firing birdshot and swinging knives and sticks, according to security officials. At least 11 protesters were wounded in the pre-dawn attack, according to a Health Ministry spokesman quoted by the official MENA news agency.

CBC Middle East correspondent Sasa Petricic said he was watching Tahrir Square shortly after midnight when a group of masked gunman started firing at the tent village set up in the middle of the square. Opposition supporters responded by throwing Molotov cocktails back, he said.

"This was a very small, very brief, fairly violent, but brief event," Petricic said. "It shows you the level of tension and the fact that is so unpredictable and uncontrollable, really by both sides."

The violence served as a stark reminder of the stakes in Egypt's political battle over the disputed draft constitution, which goes to a nationwide referendum on Saturday. The charter has deeply polarized the nation and triggered some of the worst street violence since Morsi took office in June as Egypt's first freely elected president.

"The organizers say they are trying to keep the two groups apart, but it's very very difficult to do that," Petricic said.

On one side of the divide, there is President Mohammed Morsi, his Muslim Brotherhood and ultraorthodox Salafis, while on the other there is a collection of liberals, leftists and Christians who claim the draft charter restricts freedoms and gives Islamists vast influence over the running of the country.

In Cairo's Nasr City district, a Muslim Brotherhood stronghold, tens of thousands of the president's backers, some of them waving Egyptian flags, rallied Tuesday in front of a neighbourhood mosque.

"I want the chant of 'Morsi' to shake the earth," a man on a stage set up in front of the mosque shouted into a microphone. "Alleyway to alleyway, house to house, the constitution means stability."

Crowd expanding quickly

The crowd was growing rapidly as dozens of buses, most of them bearing license plate numbers from provinces outside of Cairo, offloaded thousands of Morsi supporters at the venue. Many of them men had beards, a hallmark of Islamists, while the women wore the Muslim veil or the niqab, covering everything except the eyes.

The crowd denounced the liberal opposition and its leaders, calling them undemocratic and accusing them of being loyalists of Hosni Mubarak, the authoritarian leader who was ousted in a popular uprising last year.

An Egyptian Army tank deploys as Egyptian protesters gather outside the presidential palace during a demonstration against President Mohammed Morsi in Cairo. (Hassan Ammar/Associated Press)

"Those protesting at the presidential palace are feloul (remnants of the Mubarak regime) and counter-revolutionaries," said Mohammed Abdel-Aziz, a young Islamist protester. "They don't want Islam."

Another pro-Morsi protester, school teacher Mohammed el-Hamoul, said Islamists "accepted democracy so we could reach power."

"Now those who claim to be democracy advocates lost faith in democracy when the Islamists rose to power," he said.

Several hundred Islamists also have set up camp across town outside a media complex that is home to several independent TV networks critical of Morsi and the Brotherhood. The Islamists have threatened to storm the facility.

In the nearby Heliopolis neighbourhood, tens of thousands of opposition supporters rallied outside the presidential palace, pushing their demands that Morsi scrap the referendum on the charter.

The draft, hurriedly adopted late last month in a marathon session by a constituent assembly dominated by the president's Islamist allies, inflamed an opposition already up in arms over a presidential decree that granted Morsi near absolute powers.

The opposition responded with hundreds of thousands of the president's protesters in the streets in massive rallies — the largest from primarily secular groups since the uprising that toppled Mubarak last year. Morsi's supporters replied with huge demonstrations of their own, which led to clashes in the streets that left at least six people dead and hundreds wounded.

From there, the violence rippled across the country, with at least two dozen attacks on offices of Morsi's Muslim Brotherhood, according to the group's leaders. Meanwhile, senior opposition figures, including former lawmakers, have been badly beaten by pro-Morsi Islamists.

Referendum looming

With four days left before the referendum, the opposition has yet to decide whether to campaign for a "no" vote or call for a boycott — something many see as a reflection of divisions within the opposition. The disparate opposition groups are led by reformist and Nobel Peace prize laureate Mohamed ElBaradei, Egypt's former foreign minister and Arab League chief Amr Moussa, and leftist politician Hamdeen Sabahi.

Members of the Republican Guard stand behind a gate to block a road that protesters against President Mohammed Morsi were using to try to reach the presidential palace in Cairo on Tuesday. (Amr Abdallah Dalsh/Reuters)

Cracks in the opposition's unity first appeared last weekend when one of its leading figures, veteran opposition politician Ayman Nour, accepted an invitation by Morsi to attend a "national dialogue" meeting. On Monday, another key opposition figure, El-Sayed Badawi of the Wafd party, met Morsi at the presidential palace.

Badawi later issued a statement saying he remained loyal to the opposition's goals: scraping the draft charter and postponing the vote.

The opposition has rejected any dialogue with Morsi until he shelves the draft constitution and postpones the referendum. They had also demanded that Morsi rescind decrees giving him near absolute powers. He withdrew those powers on Saturday, but insisted that the referendum will go ahead as scheduled.

• Egypt's army given civilian arrest powers

Anticipating unrest on the day of the referendum, Morsi has ordered the military to join the police in maintaining security and protecting state institutions until after the results of the vote are announced. The decree went into effect on Monday.