The red-and-green guardhouse of the Shiite Moslem militia, Amal, at Tyre's main downtown checkpoint had a fresh coat of paint last week and was conspicuously missing a significant detail, a large portrait of Iranian leader Ayatollah Ruhollah Khomeini that for months had greeted motorists entering this ancient Mediterranean port city.

While Khomeini's likeness can still be found elsewhere in predominantly Shiite Tyre, the transformation of the guardhouse posed a subtle shift in the turbulent power politics of southern Lebanon. The Iranian-backed radical guerrilla organization, Hezbollah (Party of God) appeared to have lost ground in its struggle with mainstream Amal for dominance of the Lebanese resistance movement against the Israeli military presence in the south.

Hezbollah has mounted some of the most spectacular attacks of the guerrilla war that since last June had left six Israeli soldiers dead, 25 wounded and two captured in southern Lebanon. As recently as two months ago, which was the last time Israel-based journalists were allowed to cross the border into Lebanon, the Israeli Army command regarded the shadowy organization as potentially the greatest threat to the security of northern Israel.

A massive Israeli armored and airborne sweep of southern Lebanon in February in search of two Israeli soldiers captured by Moslem gunmen appeared to have propelled Hezbollah into new prominence in the south as the group's elusive fighters continuously engaged the Israelis in battles and then vanished in the hilly terrain.

Expressing concern over reports that hundreds of radical Shiite volunteers had moved into the village of Jibshit and several other fundamentalist villages, senior Israeli Army officers began to openly question whether Amal had the resources to prevent Hezbollah from destabilizing southern Lebanon and, ultimately, becoming a dominant force.

Senior Amal officials, including Daoud Daoud, head of the movement in southern Lebanon, seemed to reflect similar fears as they tentatively extended invitations to Hezbollah to join the resistance movement formally, presumably in hopes of exerting more control over the radicals.

Now, however, those fears appear to have diminished, and Amal, while recognizing that Hezbollah is still a rival to be reckoned with, is displaying a new assertiveness and sense of confidence in its ability to remain dominant and, as far as possible, control competing guerrilla groups.

"The effect of the Hezbollah in the south is not as strong as you think. We have some rules and regulations, and anybody who disobeys them will be punished. We are keeping security in this area, and anybody who tries to break security will be punished," Daoud said in an interview last week.

In March, Daoud said, Amal staged a demonstration in the village of Marakeh to mark the first anniversary of a bombing that left 12 Amal members dead, and 100,000 people showed up. At about the same time, he said, Hezbollah announced a demonstration and 50 turned out.

The fact that Amal can outdraw Hezbollah in southern Lebanon demonstrations is not nearly as important as the fact that Daoud, who in the past has been deferential to the Iranian-backed guerrilla force, would disparagingly call attention to it.

Moreover, Daoud and virtually every Amal official interviewed volunteered the information that at an Amal political convention in Beirut earlier this month, pro-Iranian Hezbollah sympathizers lost elections to mainstream Amal candidates by a wide margin.

One of the mainstream winners, Abdel Majid Saleh, a member of the Amal Politburo, indicated that Hezbollah would be allowed to continue to operate in southern Lebanon, but would not be formally associated with Amal.

"You can't prevent anyone from making resistance to Israel, but we have no relations with them," said Saleh.

However, Daoud made it clear that Hezbollah would have to operate within Amal-established bounds, which, he said, include a prohibition against directing Katuysha rocket fire across the Israeli border toward Jewish settlements in the northern Galilee. A number of Katyushas, attributed by Israeli officials to Hezbollah, have been fired recently, despite Amal's decree last summer that all attacks should be restricted to the "security zone" that Israel established along the border when it withdrew most of its forces in June.

"All resistance inside [the zone] is allowed, but to shoot Katyushas to Israel is not allowed. Others say we are trying to prevent resistance, but this is not so. Shooting into Israel only weakens the resistance inside, and we don't want to provide the Israelis with an excuse to invade like they did in 1982," Daoud said.

He said the only weapons that should be used in the fight with the Israeli Army and the predominantly Christian, Israeli-controlled South Lebanon Army are land mines, rocket-propelled grenade launchers and Kalashnikov assault rifles.

In the last analysis, Daoud said, the most effective counter to extremist fundamentalist groups would be a total Israeli withdrawal from southern Lebanon.

"As long as Israel stays, extremists will survive and stay, because Israel is the reason they are here," the Amal leader in the south said.

Officials of the U.N. Interim Force in Lebanon (UNIFIL) noted that since the arrival of the fundamentalist guerrillas from the Bekaa Valley, Amal now has three forces with which to contend -- Hezbollah, the steadily growing Palestine Liberation Organization contingents that have returned to refugee camps in Sidon, and the South Lebanon Army, which not only controls the security zone but the strategic crossroads at Jezzin as well.

Considering that its forces have been thinly spread to control these groups, and that it is deeply engaged itself in the fighting in the security zone, Amal has been as effective as could be expected. It also has demonstrated that it has the potential of maintaining security in southern Lebanon should Israel withdraw its presence, a UNIFIL source said.

However, the Israeli government's coordinator for Lebanon affairs, Uri Lubrani, said he expected Hezbollah and the PLO to continue to attempt to destabilize southern Lebanon and undermine Amal's effectiveness.

"The Hezbollah is trying to penetrate into Amal. They are trying to create provocations which would inevitably create unrest. If they provoke and we retaliate against the wrong people, it would create antagonisms that would serve their purpose," Lubrani said in an interview in his Tel Aviv office.

Accusing Hezbollah and the PLO of "fishing in troubled waters," Lubrani said Israel had evidence the two groups had initiated "coproductions" in the security zone against the Israeli Army, using mixed guerrilla squads. Unlike Amal, Lubrani said, "both organizations regard the welfare of the local population as the least of their worries. Their aim is to hit Israel and liberate Jerusalem."

Given the operating restrictions imposed on UNIFIL by the U.N. Security Council mandate, under which the peace-keepers cannot arrest, detain, try or punish armed guerrillas in southern Lebanon, Amal could ultimately prove to be the most effective security force in the region, Lubrani said.

"We perceive Amal as representing the mainstream of the Shiites, and we hope the time will come when we can deal with them. We are looking for a partner in Lebanon, and Amal is a potential partner," he said.

Lubrani added, "If we had a viable partner in the south, we would reconsider our deployment. Amal knows our attitude, and we just have to hope. I hope it will come off. It will take time, but I'm optimistic."

However, Lubrani stressed, if Israel withdrew now without having made adequate security arrangements, "we would be back in Lebanon in two months in force."