A Syrian-supervised security plan went into effect here without incident today, turning strife-torn west Beirut into an unusually calm city as many Moslem militiamen switched, at least for the time being, into the role of ordinary citizens.

In another move aimed at bringing peace to the strife-torn Lebanese capital, rival Christian groups announced a merger of military branches as a first step toward unifying their splintered ranks.

Lebanese policemen wearing red berets, backed by 400 Lebanese Army troops, patrolled the streets of Moslem west Beirut as gunmen put away their weapons in accordance with an agreement reached in Damascus last week.

The security plan, which also called for the shutdown of many militia offices and removal of political support from fighters disrupting public order, went smoothly. A coordinating committee including Syrian military observers toured the city as barricades were dismantled and areas were cleared of armed elements.

In east Beirut, which in recent years has had far less internal violence than has the west, the Christian Phalange Party and the more militant Lebanese Forces, the Christian militia, declared a merger of their security units after three months of bitter feuding.

Following a March 12 uprising by hard-line Christian militia commanders that toppled the moderate leadership of the Lebanese Forces and challenged the traditional authority of the Phalange Party, the Christian camp split.

Deteriorating security in Christian areas and hushed-up shootouts between loyalists of President Amin Gemayel and dissidents has recently marred stability in Christian areas, where the population had been otherwise spared the turmoil affecting much of the country.

Alfred Madi, a senior Phalange Party official, explained that the merger was aimed at forging a more coherent strategy in presenting the Christian viewpoint in future negotiations with Syria, the United States or others.

"How can we as Christians talk to anyone when we are splintered into factions?" he asked.

A televised speech by Druze leader Walid Jumblatt to his militiamen over the weekend appears to have jarred the Christians. Jumblatt said there had to be a victor and a vanquished in Lebanon and that there would never be a settlement with the Christians as long as the Phalange Party and Gemayel remained in power. "Either they kill us or we kill them," he said bluntly.

The motivation for patching up internal differences appears to stem not only from a desire to restore order in Christian areas but also from the reluctance by the feuding groups, regardless how divided they are, to have Syria sort things out between them.

Following laborious discussions in Damascus last week to reconcile various Moslem and Druze factions, a suggestion was leaked to the press that Syria was willing to host a similar conference of Christians.

In May, Elie Hobeika, 28, the new Christian strongman and chief of the executive committee that now administers the Lebanese Forces, welcomed closer ties with Syria. But sources close to him said the Christian melitias would not approve of a direct Syrian role in their affairs.

"We did not want to end up sorting out our differences in Damascus," Madi has said.

Late last night, Hobeika met with the Phalange Party head, Elie Karameh, and announced that "all military party forces in free Christian areas will be merged within the military structure of the Lebanese Forces." He said the same applied to Phalange Party security units.

Karameh endorsed the content of Hobeika's statement and pointed out that their discussions focused on the need for all decisions to be applied according to Phalange Party principles and regulations of the Phalange security council.

It was not clear whether the move meant that all the Phalange militia, concentrated in the Metn hills, the Christian heartland northeast of Beirut, would be absorbed by the Lebanese Forces and brought under its command. There are an estimated 10,000 Lebanese Forces militiamen and reservists and at least 2,000 Phalange regulars loyal to Gemayel. Gemayel reportedly has built up his own guard and police force.

Phalange officials said their security and militia elements would work jointly with Lebanese Forces commanders and that efforts would be coordinated in an operations room. This was a formula devised in the days of the late Lebanese Forces commander Bashir Gemayel. The more radical Lebanese Forces are largely in control of the Kesrouan hills, to the north of the Metn region, and the coastline.

Tension was so high in recent days that known Metn regulars avoided the coastal highway so as not to be stopped at Lebanese Forces checkpoints. A shootout broke out at Golden Beach, a summer resort north of Beirut, when a Phalange official was denied entry last week.

Jumblatt's attack on the Phalange came as thousands of Christian refugees who fled their homes in the southern Chouf and Sidon area last spring remain homeless.

Homes in at least five Christian villages seized by the Druze have been leveled by bulldozers and the reason given is that Jumblatt wants to prevent Shiite Moslems from settling there.

Apprehension among Christians that continued disarray and chaos in their ranks may further add to their political and territorial losses seems to have prompted a resolve to put their house in order, according to analysts here.