Nancy Reagan and the widow of a U.S. Army officer killed by terrorists here last winter sat holding hands in a gesture of mutual comfort tonight while 300 other Americans in Paris milled about them at a party set among an exhibition of American Impressionist paintings.

On a day in which threats of violence had forced the evacuation of six American facilities around Paris, Mrs. Reagan found herself ending the day the way she had begun it--by deploring the acts of unknown terrorists.

"She was very sympathetic, very warm and easy," said Sharon Ray, whose husband, Lt. Col. Charles Ray, had been killed Jan. 18 and who had been posthumously promoted to colonel by President Reagan last Thursday.

The American military wife had met with Reagan on his first day in Paris at his request, she said, describing how he had written to her to arrange the get-together.

"He made us feel he really cared about what happened here," she said. Early today, the 850-student American school in the suburbs of St. Cloud had been the target of a bomb by a terrorist group Action Directe, sustaining what Headmaster Neale Austin estimated was $25,000 in damages. No one was injured in the blast, which happened at 1:30 Friday morning, Paris time.

Sharon Ray said her children, Julie, 18, and Mark, 15, are both enrolled there and like the rest of the student body had been ordered by Austin to stay home from school today because of the bombing.

"That doesn't start your day off very well," said Ray.

Charles Emmons, the American Embassy's counselor for administration who is in charge of security for embassy personnel in Paris, told of five other threats on the American community of that city throughout the day which resulted in the evacuation of the American College, the American Cathedral, the American Church, the American Legion Building and "a building across from the Ramada Inn."

"It wasn't bad enough to have the summit conference," said Emmons, "but we had to have all this terrorism today as well." U.S. Ambassador to France Evan Galbraith, standing with Mrs. Reagan in a receiving line in the courtyard of the graceful, old Petit Palais, told of visiting the American School to inspect the damage.

"It blew the hell out of the place," said Galbraith.

Mrs. Reagan turned to listen and the smile on her face faded. She telephoned headmaster Austin in the morning when she heard about the blast and told him, "This kind of thing should never happen" and expressed relief that no one had been injured.

The terrorist theme was sounded by the president to an assembled U.S. Embassy staff late in the afternoon when he talked to them about a subject that obviously concerned him, and cited the assassination attempt on himself more than a year ago, while referring to the death of Col. Ray on the streets of Paris and an earlier attack last fall on U.S. Charge d'Affaires Christian Chapman, who escaped injury. "But the safety of our diplomats is of paramount concern to us . . . The delicate nature of diplomacy makes many demands upon you."

Security is heavy throughout Paris and nearby Versailles, where the seven-nation economic summit conference got under way tonight with the arrival of leaders including Reagan. But if Mrs. Reagan was concerned for her husband's safety, she did not make it apparent at tonight's party. When one guest asked her if she was worried, she smiled tightly and threw up her hands, but said nothing.

"We'll be okay," Galbraith interrupted reassuringly.

Damon Smith, president of the 700-student American College, told of evacuating the Avenue Bosquet building earlier in the day after receiving a threat that a bomb would go off an hour later. "The protest is in response to President Reagan over the weekend, not to the school," Smith said, adding that as soon as Reagan leaves, the fears will ease. He described how French police have begun patrolling in unmarked cars around American facilities since Chapman was shot at last fall.

Austin told Smith about a graffito that had been scrawled on the wall of the American School that said: "Close the school during the visit of the reactionary President Reagan or else watch out."

Among tonight's guests was actress Olivia de Havilland, who told how her daughter Giselle had been at the site of the one of the recent bombings 10 minutes before it happened. She also said Chapman gave a party not long after the attack and she could not help but have an eerie feeling about going there.

Another guest, jazz pianist Memphis Slim, a Paris resident for 21 years, said it didn't matter who came to town, "they are going to manifest--they have to have something to raise hell about."

Robert Whitbread, commander of the VFW Benjamin Franklin post here, said, "There are all kinds of terrorists and this is the opportunity for them to grab the headlines."

Not all the conversation had to do with terrorism. Reagan's mission here drew some armchair analysis from from Allan Manning, a retired foreign service officer. "People admire his courage," said Manning. "He's staking his whole career on his visit to France, trying to convince the Europeans to see it our way."

Earlier in the day Nancy Reagan was the guest of honor at a luncheon given by Danielle Mitterrand at the Elysee Palace. Others there included Liliane Thorn, wife of European Community President Gaston Thorn; Pat Haig, wife of Secretary of State Alexander Haig; Joan Clark, wife of National Security Adviser William P. Clark; Carolyn Deaver, wife of deputy chief of staff Michael K. Deaver; and a large contingent from the French cultural world, including Minister of Culture Jack Lang, who sat on Mrs. Reagan's right.

A little later Mrs. Reagan toured the National Institute for Blind Youth where Louis Braille, inventor of the Braille alphabet, once studied and worked as a teacher of piano and cello. She visited a demonstration in the gymnasium and watched students working in a chemistry lab. She received a personalized message from students learning to type on Braille machines and in turn gave them messages typed in Braille. In honor of Mrs. Mitterrand, she also gave the school an American stereo machine and some records.

Last night's party was organized late in the trip's planning stages, and some guests received invitations as recently as Monday. Originally, Mrs. Reagan was to have been feted by her friend Countess Jacqueline de Ribes, but that party apparently was called off in favor of a less elite gathering.

It was a glorious Paris evening with Americans eagerly forming a line snaking into the gravel courtyard where lighted fountains played and Mrs. Reagan stood on an oriental carpet with the Galbraiths and chief of protocol Selwa Roosevelt. Inside the museum, the exhibition of American Impressionists, which had drawn a crowd of more than 100,000 in the two months it was here, was being seen for the last time. Organized by the Smithsonian Insititution Traveling Exhibition Service (SITES), it moves next to East Berlin. Its stay here was extended by a few days to coincide with the party for Mrs. Reagan that the Galbraiths gave and the embassy paid for at an undisclosed figure. But some of the food was donated, according to Sheila Tate, press secretary to the first lady.

Young culinary students in La Varenne cooking school prepared some of the dishes, which included boeuf a la mode and fresh fruit tartlets.

"I thought they ought to do something very simple, not that tortured buffet food," said Anne Willan, founder and directress, who was food editor at The Washington Star from 1964-1967. The school is located on the Left Bank just across the Seine from the Petit Palais, and according to Willan, her students wanted to walk over with the food but "I said you're crazy in this heat."

She told how White House social secretary Mabel (Muffie) Brandon arrived at the school one morning unannounced and told her that she thought it was a good idea to showcase the talents of young American chefs.

When Nancy Reagan arrived at the party, she was introduced to the small group of student chefs who come from all over the United States and quickly spotted a tart made with kiwi fruit. She told them she wished she could try some of that a little later.

And a little later, as Mrs. Reagan sat with de Havilland, Walter Annenberg's sister Janet Hooker, the Galbraiths and Nicole Salinger, she got her wish. They wanted to give her an entire tart to take back to the U.S. Embassy Residence, but when told that there might be security problems, settled for a slender wedge, which Mrs. Reagan ate.