It was a shocking image in the devoutly Buddhist nation. The monks were refusing to receive alms from the military rulers and their families — effectively excommunicating them from the religion that is at the core of Burmese culture.

That gesture is a key to understanding the power of the rebellion that shook Myanmar last week.

The country — the former Burma — has roughly as many monks as soldiers. The military rules by force, but the monks retain ultimate moral authority. The lowliest soldier depends on them for spiritual approval, and even the highest generals have felt a need to honor the clerical establishment. They claim to rule in its name.

Instead, the monks drew on a different and more fundamental bond with Myanmar’s population, leading huge demonstrations after the government tried to repress protests that began a month ago over a rise in fuel prices.

By last week, the country’s two largest and most established institutions were confronting each other, the monkhood and the military, both about 400,000 strong, both made up of young men, mostly from the poorer classes, who could well be brothers. Rejected by both its spiritual and popular bases, the junta that has ruled for 19 years had little to fall back on but force.

It unleashed its troops to shoot, beat, arrest and humiliate the men in brick-red robes, definitively alienating itself from the clergy whose support gives it legitimacy. Soldiers surrounded monasteries, preventing monks from leading further demonstrations — or from making their morning rounds to collect the alms that feed them.

In Myanmar and other Buddhist nations, many join the monkhood as a lifelong vocation, but many other young men become monks for shorter periods, ranging from a few months to a few years. These young monks remain closer to the lives and concerns of the people whose alms they receive.

Burmese monks have taken part in protests in the past, against British colonial rule and against a half-century of rule by military dictatorship. The most notable recent occasion was in 1990.

Their militant resistance to the British produced the most prominent political martyr of Burmese Buddhism, U Wisara, who died in prison in 1929 after a 166-day hunger strike.

His statue stands near the tall, golden Shwedagon Pagoda, the country’s holiest shrine, which was a rallying point for the recent demonstrations and the scene of the first violence against the monks last week.

That attack came as a shock to people who said the military would not turn violently against the monks, and it had the predictable effect of arousing the fury of a devout population.

But monks have not always been in the political front lines. It was students, for example, who led the mass demonstrations of 1988 that brought the current junta to power in a military massacre.

The monks’ power comes instead from their role in bestowing legitimacy on the rulers.

“Legitimacy in Burma is not about regime performance, it’s not about human rights like the West,” said Ingrid Jordt, a professor of anthropology at the University of Wisconsin at Milwaukee and an expert on Burmese Buddhism. “It is something that comes from the potency and karma bestowed by the monks. That’s why the sangha is so important to the government,” she said, referring to the Buddhist hierarchy and the spiritual status that its monks can convey. “They are actually the source of power.”

The junta has gone to great lengths to identify itself with Buddhism. Like their predecessors through the centuries, the generals have been busy building temples, supporting monasteries and carrying out religiously symbolic acts. In 1999, they regilded the spire of the Shwedagon Pagoda, which now glitters with 53 tons of gold and 4,341 diamonds on the crowning orb.

The gilding of the spire was a high-risk ploy for an unpopular regime, an act permitted only to kings and legitimate rulers. When the two-ton, seven-tier finial was added and the spire was complete, the nation held its breath, waiting for the earth to send a signal of disapproval through lightning or thunder or floods, Ms. Jordt said. But nature remained indifferent.

“Aung pyi!” the generals shouted. “We won!”

But their grip on power has never been secure. They have ruled through a security service that keeps order through intimidation. They have arrested thousands of political prisoners and have held the pro-democracy leader, Daw Aung San Suu Kyi, under house arrest for 12 of the last 18 years.

In that context, the huge street demonstrations were an act of courage and catharsis.

They started tentatively on Aug. 19 after a fuel price increase raised the costs of transportation and basic goods. Veterans of the student demonstrations of 1988 staged small protests, but most were quickly arrested or driven into hiding. The unrest was fading when security officers beat monks and fired shots into the air during a confrontation in the city of Pakokku on Sept. 5.

That became a spark that grew into a broad-based challenge to the government, culminating last week in the breach between those who hold moral authority and those who have the guns.

“This was not an accidental uprising,” said Zin Linn, a former editor and political prisoner who is now information minister for the National Coalition Government of the Union of Burma, an exile opposition group based in Washington. The transition in leadership in the protests — from militant former students to activist monks — was well planned, he said, through secret meetings among young men sharing similar grievances and aspirations for their country. For the most part, it was not the elders who backed the protests. Over the years, the junta has worked to co-opt the Buddhist hierarchy, placing chosen men in key positions just as they have done in every other institution, angering and alienating the younger monks.

After the military clampdown on the monasteries last week, the streets of Yangon were mostly empty of monks. But their gesture of rejection of the junta, and the junta’s violent response, had changed the dynamics of Burmese society in ways that had only begun to play out.

The junta’s action “shows how desperate they are,” Ms. Jordt said. “It shows that they are willing to do anything at this point in terms of violence. Once you’ve thrown your lot in against the monks, I think it will be impossible for the regime to go back to normal daily legitimacy.”