Israeli troops abruptly changed course today, racing a large column of their tanks and armor from the sea across the inland mountains so that they were poised by nightfall to cut off the vital east-west Beirut-to-Damascus highway and dominate the strategic Bekaa Valley bordering Syria.

The new thrust of the three-day-old invasion seemed to indicate that the Israelis were positioning themselves to fulfill three objectives at once. First, they were apparently preparing to squeeze Palestinian guerrillas dug in along the coastal mountains in a pincer movement, cutting off their escape routes inland and attacking them from the high ground behind their positions while Israeli jets and warships continued to pound them from the air and sea.

Second, the Israelis apparently were readying themselves militarily to back their demand for the withdrawal of Syrian troops and ground-to-air missiles from Lebanon.

Third, by moving to cut the country in two by control of the road to Damascus, they were placing themselves in a strong position to impose a political solution on this faction-ridden country.

One sign of the growing Palestinian awareness of the seriousness of the situation came today as the Palestine Liberation Organization leader Yasser Arafat sent an urgent message to Soviet President Leonid Brezhnev asking him to take action to halt the fighting.

In the southern town of Tyre, 48 hours of hard street fighting took place before the Israelis were able to take the town. A spokesman for the U.N. Interim Force in Lebanon (UNIFIL) reported that fires were burning "everywhere" in the town and that Israeli forces had moved northward toward Sidon, which also was in Israeli possession by nightfall.

But the main Israeli victory of the day was the lightning-like thrust of a big tank and armor column from a beachhead just to the north of Sidon across a rugged and narrow mountainous road into the heart of the Chouf region.

By 1 p.m., Western reporters who visited the area said the Israeli column, reported by Lebanese security sources to number 100 tanks and armored cars, were passing through Beit Eddine, a key junction on the main north-south road running through the Chouf.

They said part of the column had gone north toward the Damascus road while the other went west apparently to link up with Israeli ground forces besieging the Palestinian stronghold of Damour, about eight miles south of the capital.

By early evening Radio Lebanon, quoting a Syrian communique, said the Israeli column going north had reached Barouk, only about eight miles from the Damascus highway. The Syrians were reported to be shelling the column from Ain Dara, the last town before reaching the main east-west Lebanese link.

Reinforcements arriving from Syria set up defenses on the strategic road, blocking it off at the town of Mdeirej, 17 miles east of Beirut, Agence France-Presse reported.

Once the Israelis are on the highway, they will be in a position to cut off a major Palestinian escape route into Syria as well as a main supply channel for the guerrillas.

Already they are in a position from the high mountains just east of Barouk to overlook the Syrian ground-to-air missile batteries in and around Chtaura on the Damascus road.

Apparently, no Syrian or Palestinian forces are in the Chouf region, which is the stronghold of Lebanese leftist leader Walid Jumblatt, who is currently out of the country.

The Israelis, apparently aware of this, have taken advantage of the "Chouf Gap" to penetrate with lightning speed all the way to the Damascus road.

At the same time, there were unconfirmed reports that another Israeli armored column was advancing northward from Jazzin in the south along the main north-south Chouf road leading to Beit Eddine.

Jazzin is the main center in the south of Syrian troops serving in Lebanon within the Arab peace-keeping force.

Apparently in keeping with Israeli Prime Minister Menachem Begin's statement that Israel does not seek to engage Syrian forces, the Israelis have not tried to occupy Jazzin so far. But a Syrian communique said Israeli jets had attacked their positions there and that one of them was shot down.

It also said Syrian jets had attacked an Israeli tank concentration west of Jazzin, an action apparently taken in retaliation for the Israeli air attack early this morning. Syria says that during the day's battles, seven Israeli aircraft were shot down and that it lost three of its own jets.

Despite these clashes and the growing involvement of the Syrian Air Force in duels with Israeli jets, analysts here still believe Syrian President Hafez Assad is still trying to avoid a full-scale commitment of his armed forces to the fighting.

But as the Israelis come closer to the Damascus highway and the Syrian missiles in the Bekaa Valley, there is a growing fear of a major clash between the two forces as the Syrians seek to protect their access to the Lebanese capital as well as their positions in the valley.

The Syrians regard the valley as the natural route for an Israeli attack on Syria and have long taken extensive measures to defend it.

Meanwhile, Israeli jets and warships bombarded Palestinian strongholds in the hills and along the coastal road just to the south of the capital while their ground forces closed in around Damour, only eight miles from Beirut's outskirts.

At one point, Israeli warplanes were blasting the main coastal road only about two miles from Beirut's International Airport near Khalde.