The stiff Syrian reaction both in the air and on the ground to the Israeli invasion of Lebanon today made clear the high premium the Damascus government places on protecting the Bekaa Valley, which it has long regarded as a vulnerable corridor into its own heartland and the site for a front line of defense against any Israeli attack on Syria itself.

Indications were today that virtually all the hard Syrian fighting was part of a strategy to protect the Bekaa Valley rather than the increasingly hard-pressed Palestinian guerrillas, whose forces are concentrated in the mountains nearer to the Mediterranean coast.

Syria today began pulling its peace-keeping forces out of Beirut, six years after they moved into the capital city to enforce an end to Lebanon's bloody 1975-76 civil war, and moving them to the Bekaa in an action being widely interpreted here as the end of Syrian hegemony over Lebanese politics.

A chief function of the Syrian force had been to prevent new bloodshed between the Palestinians and the Lebanese Christians, who have had close ties with Israel in the past.

But while Syria put 60 of its warplanes into the air to protect its Bekaa Valley defenses against an Israeli raid today, it has sent no such armada of planes to intercept Israeli jets that have been pounding Palestinian strongholds daily since last Friday.

An indication of Palestinian anger over Syria's failure to give them more than symbolic help came today at a press conference held by the Palestine Liberation Organization's number two official, Saleh Khalif, also known as Abu Iyad. When asked if the Syrians had done enough, he pulled on his cigar and replied tersely, "No comment."

There were reports that at least one new division of Syrian troops had crossed into Lebanon in the past 24 hours, taking up defensive positions in the Bekaa, which extends north and south through most of the eastern half of Lebanon, between two mountain ranges and roughly halfway between Beirut and Damascus.

Despite the lightning Israeli push up the coast to the gates of Beirut, it is clear that the invaders are meeting increasingly stiff resistance from Syrian forces in both the Bekaa Valley and the mountainous Chouf region between the Bekaa and the coast and had failed to reach the strategic east-west Beirut-to-Damascus highway, which remained open.

While Syrian troops and leftist Lebanese militia blocked an Israeli armor column on a twisting mountain road four miles south of the highway, Syria also reported that it had stopped a three-pronged Israeli advance in the hilly, boulder-strewn terrain at the southern end of the Bekaa, destroying 24 Israeli tanks.

Meanwhile the Syrians continued to pour fresh armor and troops into the Bekaa and one report said the Syrian forces now numbered close to 60,000, a total that would be more than one-fourth of its regular army of 222,500.

While this report could not be confirmed, Western diplomatic sources said at least one new division of troops had been added to 23,000 Syrians thought to be already here as part of the Arab peacekeeping force.

The bulk of them were already stationed in the Bekaa before the Israeli invasion.

Most of the new Syrian troops were being deployed today along the eastern side of the Bekaa, at the Syrian border. Reporters who visited the area saw them digging in close to the Lebanese-Syrian border post while tanks and armor were stationed all along the road across the valley.

Some of the Syrian troops, including the elite special forces, were being spread out along the Damascus road and around the key road junction at Mdeirej, which the Israeli armored column coming from the south is trying to capture.

But Syrian defenses along the road appear relatively light except on the eastern side of the Bekaa.

Meanwhile, there were growing indications tonight that Israel was gathering its ground forces for a strike into Beirut.

In late afternoon, several reporters came across an Israeli unit consisting of three armored personnel carriers and 40 to 50 soldiers at Doha, just outside the Beirut International Airport, three miles from central Beirut.

Western diplomatic sources said there were eight Israeli warships off the coast near Doha and Palestinians said the Israelis were bringing tanks and armor ashore by helicopter from the ships.

It was not known how many troops were aboard the Israeli vessels but any attack on the capital itself would presumably require a large force. Another possibility, according to observers, is that Israel will seek to eliminate Palestinian guerrilla concentrations in the southern outskirts of the city without going into the center.

A large Israeli column is working its way up the coastal road from the south and advance units are now fighting in and around Damour, which is just eight miles south of the city and a few miles from the unit sighted in Doha tonight.

The Israelis seem to be continuing a process of leapfrogging up the coast, going around pockets of resistance when they occur, to keep up the momentum of the invasion and give the impression of a series of rapid victories.

But Israel has a long way to go before it digs the guerrillas out of all their positions scattered along the coastal mountain range.

The PLO, meanwhile, has said it plans a campaign of behind-the-lines guerrilla warfare against the Israeli invaders and has no intention of giving up its struggle.