''We must fight our godless rulers! The only path is the law of the Quran,'' shouted Magdy Hussein, an Islamic firebrand who has led widening street protests against Egypt's Western-backed government. ''This is our jihad. Our time is now!''

The outburst in the 1,000-year-old Al-Azhar mosque took aim at the authoritarian style of Cairo's ruling elite. But such militant cries also strike at the world's largest and most influential Islamic movement: the Muslim Brotherhood.

The Egyptian-founded group - which spawned Hamas, gave underpinnings to al-Qaida and now publicly endorses peaceful reforms - is confronting unprecedented challenges to its leadership as the Muslim world is torn asunder. Competing forces of moderates who seek engagement with the West and radicals who choose confrontation are locked in a struggle for the hearts of the world's 1.2 billion Muslims - an ideological clash of Cold War proportions.

In this conflict, the 77-year-old Brotherhood is one of the linchpins.

The group portrays itself as a pillar of moderate Islam and essential for keeping radicals at bay. But those impatient for change see something else: a geriatric jihad content to work with former opponents and betray its old dreams of Islamic rule.

Whether the Muslim Brotherhood withstands the attack on its leadership, withers or is wrenched apart will be a key test of the power of Islamic militants - including those who are fueling the Iraq insurgency, resisting a Palestinian peace deal with Israel and calling for new terrorist attacks against the West.

Pitted against them, in Egypt and around the world, are mainstream Muslims convinced Islam offers the foundations for pluralistic societies - with greater rights for women and tolerance of other faiths - and who view Western nations as potential partners rather than dire enemies.

The stakes couldn't be higher.

''If the Brotherhood falters - as it represents the mainstream of nonviolent, political Islam - the repercussions are serious indeed,'' said Hugh Roberts, director of the North Africa office of the International Crisis Group, a global affairs watchdog that issued a report in March about trends in Islam. ''This will leave the territory open to fundamentalist and violent outlooks, which will become the only show in town.''

Irony is thick: The streets of Cairo, once the bedrock of the Brotherhood's strength, suddenly have become a crucible for its future.

A bold, new movement called Kifaya, or ''Enough,'' intensified rallies early this year demanding an end to President Hosni Mubarak's 24-year-old regime. At first, the Brotherhood was absent.

The group eventually lumbered into action, launching its own blitz of protests that led to the arrests of hundreds of Brotherhood members, including some top organizers. The chants of its marchers carefully stuck to the group's nonviolent creed.

''No extremism, no terrorism,'' the Brotherhood followers cried.

But a blast April 7 in a popular Cairo bazaar again showed the Brotherhood is no longer the only Islamist point of reference in Egypt. The homemade bomb killed two French citizens and an American while injuring 18 others, raising fears that militants could revive attacks on Egypt's vital tourism industry as they did in the early 1990s. Two more incidents followed.

The Muslim Brotherhood condemned the attacks, calling the bazaar bombing a ''cowardly act,'' but also saying they were a ''reaction to the injustice'' of Mubarak's tight lid on dissent.

The historical irony is thick. For much of the 20th century it was the Brotherhood that moderates and Westerners feared.

The group was founded in 1928 by Hassan al-Banna, a young primary school teacher, at the apex of British colonial power. He called for Muslims to reject all Western influences.

Operating ever since out of Cairo - the center of scholarship for Sunni Islam, the faith's main branch - the Brotherhood has influenced nearly every significant modern Islamic movement and event. (Shiite Muslims are the majority only in Iran, Iraq and Bahrain.)

Among the Brotherhood's creations are the Palestinian militant faction Hamas and violent offshoots linked to the 1981 assassination of Egyptian President Anwar Sadat and to the 1993 World Trade Center bombings in New York. Some militants later merged with Osama bin Laden's al-Qaida - whose Egyptian-born second-in-command, Ayman al-Zawahri, was once a Brotherhood follower. Dozens of legal and outlawed Islamic political movements from Algeria to Indonesia also have looked to the group for inspiration.

Then, in the 1990s, the group took a momentous turn: publicly scorning any support of violence against Muslim governments and reaching a tacit pact with Egyptian authorities, who officially still ban the Brotherhood but allow it some breathing room.

This go-slow approach is a hard sell against the fervent calls from radicals, who speak directly to legions of young Muslims.

''The Muslim Brothers is not going to disappear, but it could certainly be forced to change some aspects. There are credibility issues at stake,'' said Emad Shahin, a professor of political affairs at the American University in Cairo. ''It must be careful it doesn't lose touch with the streets.''

That's particularly true this year, as Egypt moves toward presidential and parliamentary elections. Some estimates place Brotherhood sympathizers at a quarter of Egypt's 71 million people.

In late February, 77-year-old Mubarak announced a surprise decision to allow a multiparty presidential ballot in September. While no one expects him to permit a serious challenger, the election might force the Brotherhood to get specific about what it would do on such issues as Islamic law, women's rights and the sluggish economy if it ever came to power.

Brotherhood supporters, sitting as independents, now hold 13 seats in the 454-seat legislature. They are the biggest opposition bloc, but rarely make sharp attacks on the government. The next parliamentary elections are planned for November.

The Brotherhood seems content to maintain its foothold in the system and keep its platform hazy. Abdel Monim Abdul Fotooh, a Brotherhood leader who has been jailed a total of six years in Egypt, even concedes that the Brotherhood ''doesn't want to rule now.''

That may not be enough for impatient youth - the core of protesters against Mubarak.

In a teahouse overlooking the Nile, a 25-year-old student, Abdel Mohammad Abdel, says the Brotherhood's old guard has ''become tired and gray. Young people are looking for leaders for the future, not stories from the past.''

Pressure from radicals: At the Al-Ahram Center for Political and Strategic Studies in Cairo, political analyst Diaa Rashwan sees a delicate balance at work: The Egyptian government won't try to wipe out the Brotherhood lest more radical and dangerous groups replace it. And the Brotherhood leadership will continue to disavow violence and not challenge Mubarak's hold on power.

''The Egyptian regime recognizes that the Muslim Brothers are an organic and real political force in the country,'' he said. ''They can't eradicate them. They can make them weak. Eradication, however, is out of the question.''

But their nations - Saudi Arabia, Yemen and Gulf states - which thought they had a deal with militant groups ended up suffering terrorist attacks on their own soil, and Rashwan said the same could happen in Egypt.

Al-Qaida-inspired groups could take over the Brotherhood's radical niche, bringing the fight to perceived foes on their own soil - such as recent attacks in Saudi Arabia and the Gulf.

The pressure of radical Islam is pushing the Brotherhood to make some concessions.

The current leader, Mohammed Mahdi Akef, who spent 20 years of his life in jail, has endorsed the Iraqi insurgency, rejected Israel's right to exist and called the United States ''a Satan'' out to control the Islamic world.

Still, lawmaker Mohammad Moursi, a Brotherhood member, insists: ''We've decided to seek peaceful reforms. We know this is not what many people like to hear these days. They want Islamic revolution. I fear this trend will grow.''