larmed by sightings of an Israeli buildup along the border and by repeated threats from Israeli leaders, Palestinians and Lebanese are spending Easter weekend in the grip of an acute war scare.

Yasser Arafat, head of the Palestine Liberation Organization, has warned his officers and allies that Israeli forces are likely to attack within the next few days, PLO officials said. Lebanese President Elias Sarkis called in U.S. Ambassador Robert Dillon twice today to urge Washington's help in heading off a new round of bloodshed. Sarkis also summoned Soviet Ambassador Alexander Soldatov for a separate meeting.

Dillon said after his second meeting with Sarkis that the United States is in constant touch with the Israeli government and is seeking to preserve peace. In response to Lebanese reporters, however, he acknowledged that the situation has become "dangerous."

Across the border in Israel, the frontier was eerily tranquil Saturday, with no visible troop movement or other signs that an invasion was imminent. The United States urged "utmost restraint" to avoid a major crisis. Details on Page A12.

Lebanon also sent a letter to the United Nations Security Council calling the reported troop buildup a violation of U.N. accords. But it stopped short of asking for an emergency meeting of the council.

The tension here was compounded by a feeling of frustration, with the Lebanese government unable to counter the prospect of a new Israeli assault except by pleading for help from the United States, and with PLO commanders aware that their weaponry is no match for Israeli warplanes and armor.

Tensions have been mounting since early in the year, and there have been several false alarms. But various signs in recent days have convinced a growing number of Palestinian, Lebanese and foreign officials that this time the attack is really imminent.

"They will do it," said a well-informed European ambassador. "It is coming."

Col. Azmeh Sghaiyer, PLO commander in the Tyre region 12 miles north of the border, said in an interview yesterday that his observers have spotted unusual Israeli troop and armor concentrations near Marjayoun and Blat. Both Lebanese villages lie inside the Israeli-controlled Christian enclave of Lebanese Maj. Saad Haddad and directly across from major Palestinian artillery and rocket posts near Nabatiyeh and a key PLO observation point in Beaufort Castle.

U.N. sources reported sighting large numbers of Israeli tanks on roads just south of the border and inside the Haddad area, where Israeli vehicles move freely. Sarkis was quoted by a spokesman as saying after his first meeting with Dillon that his government had information that two Israeli divisions were poised along the border.

Sghaiyer, in a conversation at his headquarters in the Rachidiyeh refugee camp near Tyre, said the number of Israeli helicopter patrols has increased in recent days over the PLO-controlled city and nearby guerrilla gun emplacements. For 90 minutes Thursday night, he said, a "provocation reconnaissance" helicopter fluttered up and down the coast next to Rachidiyeh. A PLO sentinel was heard reporting by radio the presence of an Israeli gunboat heading south a mile offshore about 2 p.m. yesterday.

An artillery shell that crashed into the village of Ain Baal, west of Tyre, injuring one person, broke an otherwise complete calm that hung over the PLO-controlled border area during a tour yesterday. News agency reports from Israel said one of Haddad's Israeli-supplied tanks was blown up shortly before the shelling by a land mine planted, Haddad charged, by Palestinian infiltrators.

Such exchanges, along with the reported Israeli sea and air patrols, appeared to observers to violate the cease-fire set up by special U.S. envoy Philip C. Habib in July.

Fears have been growing steadily since the killing of an Israeli diplomat in Paris April 3, a slaying Israeli officials blamed on the PLO despite repeated PLO denials. Radical PLO factions, however, have claimed at least two recent grenade attacks against Israeli soldiers in the occupied Gaza Strip.

Those claims ran against the grain of earlier PLO efforts to avoid giving any appearance of violating the cease-fire. Even though he insists the cease-fire does not rule out such attacks inside Israeli-held territory, Arafat had been eager to display to the world his ability to ensure compliance with the agreement.

Palestinian sources said the claims reflected a bitter debate inside the PLO in which some guerrilla leaders were questioning the wisdom of an agreement that tied PLO hands militarily while Israeli soldiers were putting down Palestinian demonstrations on the occupied West Bank.

Syrian President Hafez Assad of Syria, whose 22,000 troops here give him strong influence in PLO councils, is reportedly backing Arafat against other PLO factions.

Diplomatic sources said Syrian troops have pulled back from their southernmost positions just north of Nabatiyeh in recent days, apparently to avoid getting caught up in any Israeli attack.

There has been speculation that an Israeli attack would include an air strike against Syria's Soviet-supplied antiaircraft batteries in eastern Lebanon's Bekaa Valley. To counter that possibility, a Syrian official was quoted in a Syrian-backed Beirut newspaper today as saying the Soviet Union has offered Assad unspecified guarantees of protection for Syrian forces in Lebanon as well as in Syria itself.

After an inspection tour of the border region yesterday, Arafat told his staff that he expected the Israeli attack would center on four targets, PLO officials reported. These were PLO artillery and rocket concentrations in the hills and groves north of Nabatiyeh, the PLO-controlled Tyre region, the Damour area just south of Beirut where guerrilla training camps and weapons are located and PLO offices in Beirut.

Large-scale attacks on Tyre, Damour and Beirut would likely result in civilian casualties. Israeli's air raid last June on PLO headquarters in Beirut took nearly 300 lives, a majority of them reportedly civilians. Sghaiyer indicated northern Israeli settlements would be hit by PLO rockets in retaliation.

"If the Israelis attack, we will respond with all our power against their settlements," he said.