alestinian officials here said today their forces were putting up a stiff fight against the two-day-old Israeli offensive in southern Lebanon.

The official PLO news agency, WAFA, cast doubt on Israeli reports that their forces had occupied large areas, saying: "In reality . . . enemy forces are only in control of a few junctions along the main roads, while joint Palestinian-Lebanese leftist forces maintain their control of key positions behind enemy lines." It said the joint forces were conducting guerrilla warfare against the Israeli troops.

"We have lost positions but not forces in the field or the will to fight," one high Palestinian official said. "We've kicked them right back."

Palestinian communiques reported fierce fighting under way in the early afternoon around the 12th century Beaufort Castle overlooking much of northern Israel as well as in the coastal town of Tyre and the inland Palestinian center of Nabatiyah, about eight miles north of the Israeli border.

But there was no official word from the Palestinians later in the day about the fate of these three strongholds, and the dramatic flight by helicopter of Israeli Prime Minister Menachem Begin to Beaufort Castle in midafternoon seemed convincing proof that at least this key Palestinian site was firmly in Israeli hands.

Later tonight, PLO sources conceded the loss of Nabatiyah and Beaufort Castle. They put the number of Israeli invaders at 45,000.

One high PLO official, in an apparent sign of the growing stress on the guerrillas, asked why nothing was being done by outside powers to put a halt to the Israeli invasion.

A WAFA statement said tonight, "It is now clear that with overwhelming military superiority and unlimited U.S. backing, Israel has the capability to reach almost any territorial objective it desires. The only limiting factors are time, cost in Israeli lives and external intervention. Thanks to the prevailing Arab impotence and to international indifference or complicity, the Israelis are assured of freedom of action on the last score."

The Palestinian news agency also reported that after landing approximately 5,000 to 6,000 troops, accompanied by tanks and armor, by air and sea to the north and south of Sidon, Lebanon's third largest city, the Israelis had sent a column of undetermined size marching up the coastal road toward Damour. Tonight Palestinian sources said the column was a few miles south of Damour.

Sidon, a city of 200,000 people, was reported to be almost ringed by Israeli forces who early this afternoon dropped leaflets on the city calling on residents to surrender within two hours.

Reports from a variety of Lebanese and Palestinian sources said the Israelis by the end of the day had landed helicopter-borne commandos inside a parochial school compound within the city, apparently heralding the start of an Israeli ground assault.

The capture of Tyre, farther to the south and in the attempted seizure of Sidon marked a sharp departure from Israel's tactics in the previous 1978 invasion of southern Lebanon, when it deliberately avoided storming either of these two cities because of the risk of taking high casualties.

Analysts here speculated that the Israelis might be planning to use the city as a bargaining chip in the hard negotiations certain to take place after the end of hostilities over which Lebanese or other faction will control the south.

To all appearances, the Israelis seem to be closing a gigantic trap on the 15,000 Palestinian guerrillas now fighting in the south. Taking a lesson from the 1978 invasion, they have this time tried to block off Palestinian escape routes to the north to encircle and presumably kill or capture the largest possible number.

Already, the Israelis are reportedly estimating Palestinian casualties at around 2,500 since the invasion began, while the guerrillas are saying 400 Israelis have now been killed or wounded in the two-day ground invasion.

While no independent confirmation of either figure was possible today, there was every indication from reports reaching here from the south that the two sides were engaged in tough fighting on a number of fronts with possibly high casualties.

Among the other tactics the Israelis were reported to be using against the Palestinian guerrillas was the leapfrogging of some of their positions by the use of helicopters to lift troops and armored vehicles.

Meanwhile, Israeli jets again streaked in waves across Beirut to rocket PLO offices and headquarter buildings near the Arab university.

PLO sources at the site said the Israeli jets, using rockets rather than heavy bombs as they did in their attacks last July on the same area, had caused heavy damage to several apartment buildings.

Other Palestinian sources said one of the offices hit was the personal security headquarters of PLO leader Yasser Arafat. Yesterday he was in Saudi Arabia involved in the Islamic world's mission to end the Iranian-Iraqi war, and it was not known whether he had returned to Beirut today or was in the area.

Reporters visiting the PLO site this afternoon saw dozens of ambulances racing injured persons to the hospital, and guerrilla sources said later at least 60 persons were injured in the Israeli air raid.

Meanwhile, the city of Beirut was plunged into darkness tonight as most electricity was cut off, apparently as a result of fighting around the town of Jiyyeh, just south of Damour, where the capital's major power plant is located.

Beirut's international airport was also closed for most of the afternoon as Palestinian guerrillas fired their antiaircraft guns all across the city at high flying Israeli jets. Apparently none was hit.