Lebanese policemen and soldiers fanned out in the chaotic Moslem-dominated western sector of Beirut today in the latest drive to provide security for civilians and to bring unruly militias under control.

Joint forces of militiamen cleared some streets after 400 Lebanese Army soldiers and 450 gendarmes, wearing new gray uniforms to distinguish them from the militiamen in combat fatigues, began patrols at 5 a.m.

Many residents expressed skepticism about the small force's ability to end the reign of gunmen and halt kidnapings, bank robberies and car thefts, but several said they hoped for the best. About 40 Syrian military observers are supervising the security plan, which took three weeks of preparation.

A similar Syrian-sponsored plan introduced last July quickly collapsed.

A fierce four-day street war between Druze fighters of the Progressive Socialist Party and the Shiite Moslem Amal militia last month ended with the deployment of a joint task force of 300 men. Syria, the main power-broker in Lebanon, made it clear it would not tolerate more fighting between its local allies and began preparing the plan that went into effect today.

Since late last month, Amal and the Progressive Socialist Party, the most powerful Moslem militias in West Beirut, have sought to cleanse their own ranks by killing members they could no longer discipline and putting senior commanders in jail.

The joint Druze-Amal force will have a role similar to military police and is expected to check violations by its own fighters in order to facilitate the mission of the Lebanese Army and police.

Several branch militia offices are to be closed under the new security plan, but head offices will be maintained. The security plan does not cover the southern suburbs, home of most Shiite militiamen.

Today, Mohammed Itani, a West Beirut hairdresser, stood outside his salon watching the patrols and complained that he has had to replace his windowpanes three times since summer because of fighting and car bombs.

"And will new uniforms change anything as long as all these gangs keep their weapons?" he asked.

A Druze militia commander in charge of a seafront area, however, complained that about half his arsenal was taken away from him overnight and that he had been left only with two rocket-propelled grenades and 40 out of 100 Kalashnikov assault rifles. "I am worried about how we will defend ourselves in the next round when they come to attack us," he said.

Combat broke out between Amal and Druze militiamen Nov. 21 after the Druze went around Beirut tearing down Lebanese flags during preparations for Independence Day celebrations. Druze leader Walid Jumblatt has banned the traditional Lebanese flag in areas under his control. The flag quarrel escalated into combat with rocket launchers, shoulder-fired missiles and tanks.

No one made any territorial gains in the November fighting, the sixth showdown between Druze and Moslem militias this year. Waged from combat positions taken in apartment blocks, schools and shopping districts, the fighting killed at least 60 people and wounded 300.

There are fears that the first snags in the security plan will appear Thursday when a militia-supported strike goes into effect. Lebanon's trade unions have called for a one-day strike to protest the planned removal of state subsidies of fuel prices unless this is accompanied by significant pay increases.