The three Western nations of the peace-keeping force here were reported today to have expressed concern to Lebanese President Amin Gemayel over the alleged arbitrary arrest and poor treatment of hundreds of Palestinians during the past two weeks.

The crackdown took a dramatic turn today as the Lebanese Army, aided by French troops, sealed off the downtown area of West Beirut with tanks, armored cars and soldiers. It was a massive display of new-found authority following the Israeli withdrawal from the capital and the end of rule by independent militias here.

Western diplomatic sources said the ambassadors of the United States, France and Italy met jointly with Gemayel Saturday to discuss the arrests by the Army, part of a crackdown on illegal residents in the capital.

The sources said the ambassadors did not question the decision to make the arrests but noted their distress at reports about the way in which they were carried out and the conditions in which the Palestinians were being held pending interrogation.

Commercial activity was paralyzed in West Beirut today, with no one allowed in or out of the area as the Army conducted a house-to-house search for arms caches, criminals and aliens without proper papers. Security sources said later that 450 persons were rounded up, some of whom were seen being driven away, blindfolded with their own shirts.

Since the wave of arrests began two weeks ago, 1,500 to 2,000 persons are thought to have been apprehended. An unspecified number were deported for having out-of-date residence permits or false identity papers. It was not known if Palestinians were among them.

Italian Ambassador Franco Ottieri expressed optimism yesterday that as a result of the meeting Saturday with Gemayel, the Army would arrest Palestinians and other foreigners with proper safeguards. He said he thought the situation was "stabilizing . . . things are going in the right direction."

But today, Ottieri held urgent consultations with U.S. Ambassadsor Robert Dillon after the scope of the operation in West Beirut became clear. The results of their talk were not immediately known.

Ottieri said in an interview yesterday that the peace-keeping force was in a delicate diplomatic position because it did not want to challenge the government's right to make the arrests or undermine its authority, because one of the force's primary tasks was to to help Beirut strengthen its hold.

On the other hand, he said, the manner in which the crackdown was being carried out in the Palestinian camps and in the presence of Italian and French troops was an embarrassment to the peace-keeping force -- which is supposed to be assuring the security of the civilian population there.

The force, made up of 3,800 American, French and Italian troops, was rushed to West Beirut late last month following the massacre of hundreds of Palestinians by right-wing Christian militiamen. The wave of arrests has renewed fears among the 500,000 Palestinian civilians living in Lebanon that it is part of a larger campaign to drive them out of the country.

The Associated Press quoted Gemayel as telling the Saudi magazine Al Yamamah: "Abundant numbers of Palestinians entered Lebanon illegally of late, carrying passports from several Arab countries. Neither law nor logic allows them to remain in Lebanon. It is unacceptable that Lebanon alone bears all the burdens of the Palestinian cause."

Many Palestinian detainees have been held for more than a week before their papers were checked. Some who were then released have reported that conditions of detention, in Army barracks, were poor and their treatment at the hands of the Army and security forces often rude and rough.

The Army, after years of having no role in the capital or elsewhere in the country, has been reasserting its authority -- often with a vengeance. Its first task here has been to hunt for arms left behind by Moslem militias and the evacuated Palestinian guerrillas.

The French and Italians have taken a different position regarding their role in carrying out police duties alongside the Lebanese Army. Ottieri said Italian troops, under orders from his government, were forbidden to become involved in searching homes for arms or checking cars at roadblocks.

Their main duty, he said, was to assure that Christian militiamen or other gunmen did not infiltrate the Palestinian camps to cause more trouble and to put pressure by virtue of their presence on the Lebanese Army and other security authorities to carry out their searches and arrests in a correct manner.

The French troops, on the other hand, are working alongside the Lebanese Army at checkpoints throughout the capital, helping to check papers. Today, French troops at one roadblock were searching cars for concealed weapons while the Lebanese Army rounded up Lebanese and aliens without proper papers.

U.S. Marines, based at the airport and outside populated areas of the city, are not involved in providing security at the camps or in helping the Lebanese Army reestablish authority in the capital.

The Los Angeles Times added from Beirut:

Newly reappointed Premier Shafiq Wazzan said in an interview that Lebanon would not enter into a peace treaty with Israel. In the Arabic magazine Kul Al Arab, Wazzan said Lebanon would sign such a treaty only as part of an overall settlement of the Middle East question. A treaty between Lebanon and Israel under the prevailing "conditions of conquest" would only cause problems, he said.

Meanwhile, the Palestine Liberation Organization's representative in Lebanon, Shafik Hout, has sent condolences to Gemayel on the death of his brother, Bashir. who was assassinated Sept. 14.

The condolences came as something of a surprise, because the Gemayels and the PLO have been bitter enemies. The message could be an attempt to patch up differences.

In another conciliatory move, Saudi Arabian Ambassador Ali Shaer paid an official visit to Gemayel. The Saudis have long been supporters of the PLO, even when the group wrested power from the Lebanese government.