Northern Israel was put on military alert today and tight security was imposed elsewhere in the country following early morning attacks by the Israeli Army against Palestinian terrorist bases deep inside southern Lebanon.

The Israeli Army chief of staff, Lt. Gen. Rafael Eitan, said the army units marched 10 miles and waded across the Litani River to reach their destinations. Although earlier reports said mechanized vehicles and helicopters were used to transport the attackers, Eitan said the entire operation was on foot, and that no vehicles were used.

Infantry and paratroop units struck three villages north of the Litani River, blowing up terrorist training bases and destroying artillery and rocket installations before withdrawing to Israel about four hours later.

It was the largest army operation inside Lebanon since the large-scale invasion last March in retaliation for a terrorist attack on a tourist bus in which 41 persons were killed.

Palestinian guerrillas responded tonight by firing a barrage of rockets at Kiyrat Shemona, a town near the Lebanese border, driving thousands of people to underground bunkers.

The Israeli forces avoided U.N. peacekeeping forces in the attack by crossing into Lebanon at points held by friendly Lebanese Christian militiamen across from the northern finger of Israel's frontier near Metulla.

The U.S. Security Council criticized Israel for a "lack of cooperation" with the peacekeeping force as it extended the force's mandate another five months, news services reported.

[Meanwhile, both Egypt and the United States criticized the Israeli decision to move into southern Lebanon.]

Israeli military sources said about two dozen members of Al Fatah, the main military arm of the Palestine Liberation Organization, were known to have been killed in today's incursion, and that others were presumed dead under the rubble of destroyed buildings. One guerrilla was captured and flown to Israel for interrogation.

One Israeli soldier was slightly injured in the attack, Israeli officials said. In Beirut, however, Palestinian sources claimed there were about 6 Israeli casualties and that guerrillas had taken some Israeli prisoners.

The Palestinians said at least 21 guerrillas were killed and 13 wounded. They reported five Palestinians missing and 25 Lebanese leftists captured and apparently taken to Israel as prisoners.

Although the Israeli Defense Ministry said the army strike was conducted on the basis of a general policy of attacking places where terrorists train and prepare attacks on Israel, it followed closely behind a series of guerrilla bombings in Israel and a three-man Arab attack last Saturday at Maalot in northern Israel.

Four units of the Golani infantry brigade and the paratroopers caught the Fatah villages completely by surprise by slipping into Lebanon on a cloudy, moonless night, army officials said. The army would not say how many troops were involved. A fullstrength Israeli brigade usually comprises about 3,500 troops.

Military officials said some Israeli warplanes were circling in the area as a precaution, but they were not used in the strike. An artillery barrage from inside Israel was unleashed against targets north of the Fatah bases to prevent terrorist reinforcements from moving forward, officials said.

The principal targets were Arnoun, a small village near the ancient crusades castle at Beaufort that has long been a PLO stronghold; Mahmudiya, a village about 10 miles to the north, and Ayishiya, a village that lies about two miles south of the Syrian-dominated Arab peacekeeping force's southernmost outpost.

Israeli officials said the Syrian Army units were not engaged during the operation.

The army said Fatah housing and ammunition depots were blown up at Ayishiya and Mahmudiya and that observation posts, bunkers and arms depots were destroyed at Arnoun. Moreover, the army said, artillery, mortar and Soviet-made Katushya rocket bases just north of Ayishiya were attacked.

The Palestinians responded at about 6 a.m. with three mortar shells that fell near Metulla, the army said.

Defense Minister Ezer Weizman said at a news conference that the strike was "one of a series of operations aimed at preventing terrorists from attacking the civilian population.

"We have to continue to hit them in Lebanon as long as the... Lebanese government continues to permit the use of its territory" by the PLO, Weizman said.

Army chief, Eitan, in an interview in the afternoon newspaper, Yediot Ahronot, revived the continuing feud between Israel and the U.N. peacekeeping force in Lebanon by charging that the U.N. forces "accept the situation" of guerrillas operating freely in southern Lebanon.

Eitan said Fatah was becoming increasingly sophisticated, and added that he would not be surprised to see PLO units operating tanks soon. He said Fatah is already training pilots.

Following the Maalot attack, in which an Israeli woman fell to her death from a balcony trying to escape and in which all three attackers were slain, the army charged that the infiltrators crossed through U.N. positions to enter Israel. U.N. officials denied it, saying the terrorists crossed through Christian Lebanese territory.

Following the attacks, kibbutzim and military units in northern Israel were put on the alert for terrorist reprisals, and many security officers were seen in public squares, markets and other common bombing targets throughout the country.

Army officials said the recent wave of bombings in Israel probably was linked to the PLO council meeting in Damascus, and they said more Fatah attacks were likely.

"Basically, [the Lebanon incursion] does not change much, but this is a continuing struggle, and you have to change the methods," an army official said. Most recent strikes against Fatah bases in Lebanon have been from the air or from gunships off the coast.

In another development, U.S. Ambassador-at-Large Alfred Atherton met again today with Prime Minister Menachem Begin and Foreign Minister Moshe Dayan to discuss the deadlocked Middle East peace talks. Atherton and State Department legal adviser Herbert Hansell also met with a team of Isreali legal experts on the American interpretation of two disputed clauses in the compromise draft treaty between Israel and Egypt.

After his meeting, Begin said "some ideas were presented" and that differences remain, but that the government will make no decisions before Sunday's Cabinet meeting.

Atherton is scheduled to remain here until Sunday, when he flies to Cairo.