President Reagan's "unstructured" summit came to an end tonight with some unstructured protocol. Everybody seemed to love it.

At Reagan's official dinner for his summit partners there was absolutely no order to how people arrived and absolutely no discipline to how they behaved when they got there.

The biggest cut-up was Italian Prime Minister Amintore Fanfani. He started off the evening by arriving first. In the order of protocol he was seventh. He promptly set about breaking off roses to hand out to some of the other guests.

Later, at dinner, he sketched on the back of his place card, then sent it down to Reagan. That done, Fanfani started another sketch, this time of his host.

The summit heads left little doubt that they are as individual in manner as they are in politics. Canada's Pierre Elliott Trudeau, the weekend's fashion pacesetter, strolled down the tree-lined street to the Abby Aldrich Rockefeller Folk Art Center wearing a white double-breasted jacket with a rose in the lapel and dark trousers.

Another stroller ignoring protocol was West German Chancellor Helmut Kohl. But Japanese Prime Minister Yasuhiro Nakasone, the European Communities' president, Gaston Thorn, and French President Franc,ois Mitterrand opted for motorized conveyance to the party.

"Our individual perceptions about particular issues may sometimes differ," Reagan told them during his after-dinner toast. "But gatherings such as this give us an opportunity to work together on a regular basis to address the problems we share. This meeting has, in my judgment, achieved that objective."

Reagan said he felt more confident than ever of the "basic health of our free way of life and our ability and cooperation to lay a sound foundation for our children and our children's children."

He raised his glass of Schramsberg Cremant Demi-Sec 1980 "to the causes that have brought us here, to the success that we've had and to our dream of continuing on this road as far as we all can see."

The evening had everything going for it, from weather to entertainment. The latter was provided by Metropolitan Opera soprano Leontyne Price and violinist Eugene Fodor. Nancy Reagan and Barbara Bush were the only wives of summit conferees present--but not the only women. Several women in the foreign delegations attended the dinner.

Fanfani arrived even before Vice President and Mrs. Bush had a chance to get into position to welcome him. He didn't seem to mind, though, and when the Bushes did catch up to him, he broke off a yellow rose from one of the gaudily blooming bushes and handed it to Mrs. Bush.

When Trudeau arrived, Fanfani broke off another rose and gave it to him. But one rose was apparently enough for one lapel, so Trudeau gallantly gave it to Fanfani's interpreter. Fanfani's "gardening" ended before the next guest, and Thorn was without a rose.

About 8 p.m., the Reagans reached the garden. Trudeau had a kiss on each cheek for Nancy Reagan. And from Thorn and Kohl the first lady got courtly kisses on her hand. Reagan didn't seem to notice when Mitterrand arrived, and by the time he did it was too late to offer him a drink. Photographers were waiting on the other side of the pool to take the class picture.

Dinner was in a tent filled with plants and potted trees. There was American caviar, Sally Lunn, melba toast, roast rack of lamb, salad with greens, vinaigrette, boneless quail with wild rice stuffing, cream sauce, fresh asparagus, baked California goat cheese and Florida citrus sherbet, plus two California wines.

Fanfani, an artist in his spare time, picked up his place card and started to sketch. The White House said later the president had told him Italian President Sandro Pertini had been quite taken with one of the women violinists in the Army Strolling Strings during his American visit last year. So while that group entertained during dinner, Fanfani sketched her in action. That was the sketch he sent to Reagan, who sat in the middle of the table.

Later Fanfani started another sketch, apparently of Reagan.

Entertainment was under two large maple trees on the terrace of Williamsburg Inn. Potted red geraniums bordered the stage, and in the front row sat the Reagans, with Mitterrand on the first lady's right, and Trudeau on the president's left.

Leontyne Price told the audience as she introduced the program, "I've never been prouder to be an American."

Fodor performed first, selecting as his final number a work by William Kroll. Fodor said Kroll was particularly important to him since he headed the jury in 1967 that awarded him first prize in the Merriweather Post competition.

Price's opera selections included a German number and an Italian one. In addition to enthusiastic approval by the German and Italian leaders, she also drew bravos from the crowd.

Among them were Virginia Gov. Charles S. Robb and the state's two senators, John W. Warner and Paul S. Trible Jr.

Price bounded off the stage before Reagan had a chance to thank her and Fodor. But the first lady gave Price a big hug, and then the Reagans, with their summit visitors walking along beside them, strolled across the lawn to the east lounge of the inn, where they said good night.