Tens of thousands of ultraconservative Muslims in long beards, robes and prayer caps thronged Cairo's central Tahrir Square in a massive show of force Friday, calling for the implementation of strict Islamic laws and sparring with liberal activists over their visions for a post-revolution Egypt.

It was the first rally with religious overtones in Egypt, and one of the largest, since the uprising that forced President Hosni Mubarak to step down in mid-February. The strong showing by the Islamists demonstrated their powerful organizational abilities, which will likely help them in parliamentary elections later this year.

"Islamic. Islamic. Not Western or Eastern. No liberal or secular," chants of Salafis, who follow a strict form of Islam, echoed through the square. Others shouted: "With our soul and blood we defend you Islam."

They unfurled an Egyptian flag, removing the central emblem of an eagle and replacing the Islamic slogan: "There is no god but God and Muhammad is his prophet," similar to the insignia on the Saudi flag.

The youth activists who have been at the helm of mass protests calling for faster change from the country's interim military rulers withdrew from the rally soon after Friday prayers, accusing the Islamists of violating an agreement to avoid divisive issues.

"While the civil organizations are trying to respect the effort to complete the revolution by unifying the ranks, the Islamic groups insisted on breaking the unity and assisting the military council in a deal that I think will divide this country in two," said liberal activist Mustafa Shawki. "This is what we were afraid of."

Several hundred protesters, mainly liberal and leftist groups, have camped out at the square for more than three weeks, demanding swifter justice for those blamed in the killing of nearly 900 protesters during the 18-day uprising and more measures to ensure Mubarak loyalists are purged from the government. It was a crowd vocally critical of the military council, which they accused of protecting Mubarak's regime.

Most of the Islamic groups, however, say the military needs time to break with the past.

The decision by the Salafis and the Muslim Brotherhood, Egypt's best organized political force, to participate significantly boosted the turnout. But instead of a day of unity as had been advertised, the Islamists decided to flex their muscle, using the epicenter of the protests to press demands for a strict version of Islamic law.

Some Salafi Islamist groups mobilized their members to the square to oppose the adoption of a set of guidelines for drafting a new constitution after parliamentary elections later this year. Buses from a number of cities transported followers, many who were in the square for the first time.

Liberal parties are worried religious groups will win a large share of parliament and force an Islamic influence on the constitution. The Islamists say nothing should restrict the newly elected parliament's right to oversee the process of drafting the document.

"The liberals are talking about a civil state. This won't work in Egypt," said Tarek Shaheen, a 31-year-old resident of Ismailiya. "We want to prove to the outside world even before domestically that Egypt is Islamic, that it has a large Islamic trend and that we are not terrorists."

While opposing the measure, Muslim Brotherhood members did not press the issue Friday sticking to the agreement.

Salafis are ultraconservatives, close to Saudi Arabia's Wahhabi interpretation of Islam and more radical than the Brotherhood. They seek to emulate the austerity of Islam's early days and oppose a wide range of practices like intermingling of the sexes that they view as "un-Islamic." Many also reject all forms of Western cultural influence, and preach that authorities must be respected.

Mubarak's regime cracked down heavily on Islamic groups, specifically the politicized Brotherhood, arrested thousands of its members. Salafi groups are new to the political scene in Egypt.

Many like Shaheen felt that Egypt's Islamic identity is threatened, reflecting the growing mistrust between the different groups only months before the first parliamentary elections, the first after Mubarak's ouster.

Egypt's constitution, which has been suspended by the military rulers, set Islamic law as the basis for legislation and nobody has proposed changing that clause. But some Islamic groups believe the liberal groups will use the guidelines to introduce what they perceive as Western values.

"Our religion is the constitution," said Saber Mohammed, a 27-year-old Cairene wearing a short white robe and head cap, sporting the traditional bushy beard of a Salafi.

Nourhan Zamzam, a 29-year-old banker who supports the call for a civil state, said the ultraconservative Islamist groups are vying for influence but have little experience.

For her, the Salafi stance only undermines pressure on the military by dividing the efforts of the protesters.

"This is actually a message to us, the revolutionaries, who are critical of the military council," she said. "This is a message to scar us: look infighting between groups is coming."

By sundown, a large number of Islamists began leaving the square peacefully and the sit-in continued.

It was more tense in other cities.

In the southern city of Assiut, Salafist protesters beat up a group of protesters from the Communist party trying to join their demonstration, deputy police chief Yosri el-Gammasi said. At one point, some in the crowd yelled back at a speaker who criticized the idea of constitutional guidelines.

In the Sinai city of el-Arish, government troops clashed with Islamic militants firing rocket-propelled grenades and other heavy weapons outside a police station. Four people were killed, including a military officer and three civilians, and 18 people injured.

South of the capital in Minya province, gunmen fired on a car carrying Christians, killing two and injuring two, a military official said. It was the second killing in two weeks in the predominantly Christian village of Roman. While the motive was unknown, similar events have sparked religious violence in the past.

Officials spoke on condition of anonymity because they weren't authorized to brief the media.

Friday's rally came a day after Egypt's Justice Ministry said Mubarak, along with his two sons, his former security chief and seven others, will be tried Wednesday at a Cairo convention center. Mubarak, 83, faces charges of corruption and ordering the deadly use of force against protesters.