Without violence, massive numbers of Iranians today staged their biggest protest march to date against the foundering authority of Shah Mohammad Reza Pahlavi.

The carefully disciplined, well organized march through the capital went on for more than six hours and lent considerable weight to the religion-dominated opposition's claim that it represents a viable alternative to the government.

The feared bloody clashes between protesters and the shah's army never materialized. Almost all troops and police were absent from most of Tehran, expecially in areas hit hard by arsonists and rioters in earlier demonstrations.

Marshals in the street -- and turbaned Moslem clergymen using hand-held loudspeakers atop buses -- regularly reminded the crowd to avoid the violence so feared by organizers. Their appeals reflected a bargain last week whereby the government lifted a ban on marches in exchange for a pledge from the opposition to keep the protest peaceful.

Although the shah's name was not mentioned on banners or in chanted slogans, the whole day consituted an unmistakable show of no confidence in the monarch.

It was the concession Friday by the shah, reversing an earlier martial law ban on the march, that may have spared Tehran from bloodshed. Had the shah not relented and had the army split over reluctance to fire on fellow Iranians, observers are convinced he would have been under an increased threat of a military coup to drive him from the throne he has occupied for 37 years.

Estimates of the numbers of marchers varied widely. The government-controlled radio first spoke of only "tens of thousands," but later mentioned "around 400,000" while the National Front opposition claimed 2.5 to 3 million of the capital's 4.5 million residents took part.

Conservative crowd watchers suggested perhaps 750,000 marched under leaden skies.

Old men, mothers carrying babes in arms, women wearing full-length veils, laborers, white-collar workers professional men and especiall many young men took part in the march, which started in seven locations.

By the time they all funneled into the main line of march on Shah Reza Avenue for the five-mile walk to the dispersion point, at Shahyad monument commemorating 2,500 years of Iranian history, the six-lane thoroughfare was black with people.

Karim Sanjabi, the National Front leader only recently released from prison, hailed the march as proof Iranians are determined to end "dicatorship, despotism and corruption." He said the "great human tide" in Tehran was the best proof of the justice of the cause of our people" and warned those at home and aboard not to count on any "attrition" of Iran's slow-motion revolution.

Theoretically, the processions marked International Human Rights Day and the first of two days commemorating the 1,298th anniversary of the assassination of Hossein, grandson of the prophet Mohammed and founder of the Shiite sect of Islam which is Iran's state religion.

But from beginning to end, the march seemed to reflect the appeals of Ayatollah Ruhollah Khomeini. It was the name of the exiled Moslem divine that the crowds chanted time and time again. And it was his portrait they carried in great profusion, demonstrating support for his 15-year-old demands for the overthrow of the shah.

The shah's Niavaran Palace in the foothills of the Elburz Mountains was well within a ring of heavy tanks, machine gun-mounted jeeps and troops that the government set up to separate prosperous northern Tehran from the opposition crowds.

A black-clad Iranian woman named Irene, who said she had recently returned from a 12-year stay in San Francisco, summed up the mood of determined defiance and uncertainty about the future:

"It's impressive. I don't know what is going to happen. Every human being has the right to say what he thinks. It's in believing, marching like this without any guns, empty-handed."

Asked if she thought the shah had got the message, she replied: "The shah should have got the message a long time ago -- 15 years ago."

"Maybe his palace is so big he cannot hear the people," she added.

Her male companion said, "More than the shah, the Americans should get the message."

Only minutes before, a young woman holding a child had approached foreign journalists and spat out, "Death to the Americans."

Anger at American support for the shah was a constant theme in slogans on banners and in the conversations of marchers, a surprising number of whom appeared to be American-educated.

Yet, for the first time since martial law was proclaimed, no army troops guarded the American Embassy or those of other countries.

The organizers were determined not to be tarred by the military government's suggetions that they were the tools of communists. One of the few banners in English clearly made the point: "No communists -- we are Moslems."

Route marshals discouraged marchers from picking up leaflets distributed by the banned Tudeh, or Communist, Party.

A young marshal in a battered American Army surplus field jacket said, "Yes, we have arms, but in our houses. We don't need them now."

A 37-year-old doctor named Manuchehr echoed marly of those interviewed, saying, "The shah must go. We want nothing to do with the shah, with the past. We want something new."

For some, that "something new" was Khomeini's yet ill-defined Islamic democracy with which he wants to replace the 50-year-old Pahlavi rule. For others, it was simply parliamentary democracy along Western lines.

A 41-year-old factory manager who called himself Ahmed acknowledged that Iran might go through a period of confusion and anarchy if the shah left.

"In the long run it may bother me -- I have fine and my two kids are studying in the United States and Britain -- but liberty does not come to you at no cost," he said.

"It would, it may, all go up in smoke, and a lot of blood may still have to be shed. But what is one generation sacrificed in the life of a nation -- just a drop in the ocean."

But more telling than such talk were the anger and frustration against the shah expressed by marchers. Their complaints ranged from bribery and corruption to torture, repression and arbitrary imprisonment.

A banner reading "We shall never be pushed down again" seemed to capture the mood as much as the constantly repeated chant cAllah is great," which symbolized the role that Islam has played in denouncing government excesses.

Monday marks Ashura, the principal day of the holy month when Shiite Moslems traditionally have beaten their chests bloody as a sign of grief at Hoseein's death. It was unclear whether Ashura would be the occasion for further massive public demonstrations or would be restricted to smaller processions.