Women in long black shapeless chadors, the dress of Islam, walk Al Hamra, which in its heyday not very long ago used to be called the Champs d'Elysees of west Beirut.

Black banners drape city streets, summoning Moslems to sacrifice and to join the battle against Israel.

From the sides of crumbled buildings stare the images of a surging Islamic fundamentalism -- the poster faces of Iran's Ayatollah Ruhollah Khomeini and the leader of Lebanese Shiites, Imam Musa Sadr, who vanished on a trip to Libya in 1978. On many walls, amid the pockmarks left by shellfire, are scrawls of "Kuluna Khomeini" ("We are all Khomeini") and other revolutionary graffiti.

Once a center of cosmopolitan interests and the fun life, west Beirut is today increasingly in the grip of Islamic traditionalists who, some longtime residents worry, may be permanently altering the character of the city.

In the midst of the concrete rubble, scorched storefronts, shattered glass and battered cars that mark the vast physical destruction of Beirut after nearly 10 years of civil strife, a social evolution is under way, brought on by the waves of poor Shiite Moslem refugees who have poured into the capital during the past two years.

Feeling displaced, destitute and uncared for, these Shiites are turning increasingly restive and militant. A growing number are massing behind the shadowy fundamentalist movement called Hezballah, the "Party of God." Their influx and agitation have produced tensions with the moderate Sunni Moslem community, until now the dominant influence in west Beirut.

"Definitely, you can see a tendency towards an increasing Islamization of west Beirut, a growing religiosity," said Fuad Khouri, a professor of social anthropology at the American University of Beirut. "Religion was always a background for action; now it's a front."

Officials in the mainline Shiite movement, Amal, admit to being worried particularly by the growth of Hezballah, with which they are battling for the shifting alliances of Shiites.

The swelling Moslem fundamentalist presence in Beirut has been especially pronounced in the past week, during the 10-day solemn Shiite holiday of Ashura, which climaxes Friday. The holiday commemorates the martyrdom of Imam Hussein, grandson of the prophet Mohammed, in 680 at Karbala in present-day Iraq.

This year's observances have been made all the more impassioned by the continued Israeli occupation of southern Lebanon, which is heavily populated by Shiites. Large framed pictures of Moslem youths who have died in raids against Israeli forces in the south are being carried in holiday processions and prominently displayed at Shiite religious services.

Also reflecting a heightened sense of militancy, Moslem extremists are enforcing a ban on alcohol with particular vengeance. About a dozen bars and nightspots in west Beirut have been bombed or ransacked since the start of the Ashura mourning period.

In the most dramatic such reported case, more than 100 Moslem women wearing veils and black chadors went on a two-hour rampage last night, smashing liquor bottles, furniture and fittings in several boarded-up bars and bingo parlors on Phoenicia Street, once the hub of west Beirut nightlife.

This wave of unchecked violence drew a frantic denunciation from Sunni leaders this week. In a statement, they deplored a lack of security in the city, saying residents "were prey of armed bands" and were being forced to flee the western sector.

The Lebanese Army's 6th Brigade, a largely Moslem unit which broke away from the Army's Christian leadership in February and was reconstituted in July to reassume control of west Beirut, appears ineffective against the marauding bands of Moslem gunmen. At night, the Army and police have virtually stopped patrolling the streets, which areoften blacked out by power cuts.

In addition to their fear of lawlessness, Sunni residents resent the religious fervor and rigor which the extremists seek to impose. "We had a free life before," said one Sunni driver. "Now the Shiites are here and they think differently. They give orders, especially Hezballah, about drinking and dressing and other things. We're Moslem, too, but we don't like anyone giving us orders."

Yet the Sunnis seem powerless to put up a defense. Unlike the Shiites and Druze, Sunni leaders never developed their own militias.

"The Sunnis are a decadent bourgeoisie in this part of the world who inherited the glory of the Ottoman Empire," said a member of the Sunni establishment in west Beirut. "They are not warriors, not fighters. They are merchants. The Sunni leadership played its cards badly. They failed to build up their own forces."

At the same time, Sunni leaders are understood to be reluctant to confront the Shiites, for fear of fanning intra-Moslem tensions that could be exploited by opposing Christian, Israeli, Communist or other forces.

"There's a proverb we have: 'Me and my brother against my cousin, and me and my cousin against the stranger,' " said a Sunni politician. "As long as there are strangers in Lebanon, like the Israelis or others, we will not be dragged into a confrontation with our own kind."

Nonetheless, others here fear that unless something is done to restore a sense of security and respect for individual rights in this battered capital, the liberal way of life that west Beirut used to be known for will be snuffed out.

"This is the only island of multi-confessionalism you have left in Lebanon," said a Sunni establishment member, referring to the mix of Sunnis, Greek Orthodox, Christians, westerners and others who populate the western sector of the city. "If this part of the city loses this modus vivendi, you wipe out the essence of Lebanon."

For their part, the Shiite refugees are plagued by miserable living conditions. Tens of thousands of them arrived here in 1982 after the Israeli Army invaded their villages in southern Lebanon, joining others who had moved north after Israeli bombing raids in the 1970s. Tens of thousands more were pushed out of the slums south of Beirut at the start of this year when heavy shelling by the Lebanese Army made them homeless.

They have moved into abandoned apartment buildings and stores in west Beirut, often living five or 10 or more to a room. Whatever work they can find usually involves menial chores. For them, the power failures, water shortages and piled-up garbage that affect nearly everyone here are felt most acutely.

Feeling down and out, many of these refugees find a restored sense of community, of belonging to something, in religious rallies. Increasingly, too, Islamic fundamentalism here is being fueled by frustration with the lack of results produced by political leaders.

Leaders of the mainstream Amal organization blame the expansion of the Islamic fundamentalist movement in Beirut on the refusal of the Christian Phalange Party to cede some power to Lebanon's Moslem communities.

"We tried many times to explain this to the Phalange and to President Amin Gemayel, that if we don't succeed in creating a unified country, the extremists will take hold -- on both sides," said Akef Haider, an Amal leader.

Said a senior Druze official: "If Amal leader Nabih Berri and the moderates get a reasonable dose of political reform, along with some social welfare action, you will see some receding of the extremists."

Hezballah itself is said by some analysts to be a front or spearhead for several militant Shiite groups under a Syrian umbrella, with Iranian political, financial and military support. These groups are said to include: Islamic Amal, led by Hussein Mussavi, who split with Berri two years ago; Al Dawa, an Iraqi Shiite opposition party; and such other splinter organizations as Soldiers for God and the Lebanese Students Union.

Western intelligence sources have been quoted saying that Islamic extremists first infiltrated into the Beirut area last year to develop terrorist cells against western targets in Lebanon and to cow the Lebanese government into submission. Hezballah has been mentioned in connection with "Islamic Jihad," the name used by telephone callers who claimed responsibility for three devastating suicide attacks against U.S. targets here.

But interviews with Shiites in two Beirut neighborhoods who claim to be followers of Hezballah suggest the movement has acquired a broader social base.

There appears to be little formal structure to it. Adherents say there are no membership cards. They appear bound mainly by the goals of a national Islamic republic and the defeat of Israel.

Emerging partially from the shadows, Hezballah has started publishing a weekly newspaper. Its posters cover an increasing area of the city. They show a machine gun next to a Koran and an olive branch. Inscribed across the bottom of the posters are the words: "Islamic Revolution in Lebanon."

Still, no one in the Beirut area has stepped forward as the leader of Hezballah. Sayyed Mohammed Hussein Fadlallah, a 49-year-old Islamic scholar often mentioned as the movement's spiritual chief here, has repeatedly denied any direct association with Hezballah.

But Salim Hoss, a Sunni political leader who is currently minister of education, said in an interview that whenever a problem comes up with Hezballah, he telephones Fadlallah, who "has been very helpful."

In an interview in his office in the southern Beirut suburb of Bir Abed, Fadlallah attributed the growing militancy among Shiites to the general war-torn situation. He said Hezballah is "a large group" that "seems to be growing rapidly."

As a relative newcomer to the scene, the movement, he said, has not been tarnished by the political mistakes made by the Amal leadership. While Amal's Berri is a member of the current Cabinet, Hezballah continues to shun any political bargaining arrangement with other Lebanese parties.

Addressing the fears that many Lebanese have of a Shiite drive to turn the country into an Islamic republic, Fadlallah said: "I don't believe the Moslems here are trying to achieve an Islamic government in Lebanon. This is different from what Khomeini achieved in Iran. The Moslems in Lebanon just want equal rights."

Nonetheless, Fadlallah and other Shiite leaders have appeared worried lately about their followers getting out of hand. Speaking to more than 3,000 Shiites packed into an underground garage under the sign of Hezballah one night this week, Fadlallah urged the crowd to contain their zealousness.

He warned against public displays of Shiite rituals that might convey "an image of backwardness," adding: "Sometimes, instead of going out in the streets, it is better to stand in a mosque and reflect."