After three days of grappling with exchange rates, Col. Qaddafi and a Russian nuclear reactor, seven summit leaders sat down to dinner tonight with a Japanese emperor whose 60-year rule has included starting and losing a war and having two atomic bombs dropped on his country.

Emperor Hirohito, at 85 one of the last remaining links with the extraordinary history of pre- and post-war generations, hosted the ceremonial finale at the Imperial Palace. The empress, who has what the palace describes as a "bad knee," did not attend.

But Crown Prince Akihito, 52, was there, with his commoner wife, Princess Michiko, providing summit goers and their wives a chance to meet the man who will someday succeed Hirohito. So far, though, Hirohito has not indicated that he ever plans to relinquish the so-called Chrysanthemum Throne.

Until Princess Michiko underwent unexpected surgery recently, she and the prince had been scheduled to visit the United States this month. That trip has now been postponed until later in the year, the palace said tonight.

President Reagan, as the summit's ranking leader, was presented to Hirohito first. Later, at dinner in the Homeiden ("Abundant Bright") Room, Nancy Reagan sat on the emperor's right. Reagan sat across the H-shaped table from Princess Michiko. Between Reagan and France's President Franc,ois Mitterrand was Crown Prince Akihito.

The Japanese managed to solve any protocol problems they might have had involving Mitterrand, president since 1981, and French Prime Minister Jacques Chirac, the opposition party leader who took office in March, by putting Chirac some distance down the table but in a prime spot next to Prime Minister Yasuhiro Nakasone.

As he usually does at these events, Mitterrand wore his statesman smile. Since Chirac is a newcomer, his was more of a just-plain-Jack smile. He wore it when he arrived at the palace, striding in to shake hands with the protocol chief, whom he apparently mistook for Crown Prince Akihito. He wore it again when he spotted reporters inside the huge banquet hall, where a gigantic tapestry depicting aqua, peach, violet and flame-colored clouds monopolized an entire wall.

There were no chopsticks at the table because the menu was "international," according to the palace, featuring swallow's nest soup, fish stewed in sake, roast leg of lamb and ice cream shaped like Mount Fuji.

The beverages were also international -- Canadian whiskey, American bourbon, West German Rhine wine, a French bordeaux, British Scotch, Italian vermouth and Japanese sake.

There weren't, however, any toasts -- not even to the emperor -- because of the protocol complications and the Japanese desire to avoid any breaches.

Typically, the Japanese arranged spectacular bouquets for the tables, this time of peonies, and delayed the blooming of two flowering cherry trees so they would blossom for the banquet. They sat in tubs in two corners of the room.

Besides Reagan, Italy's Bettino Craxi, Canada's Brian Mulroney and West Germany's Helmut Kohl brought their wives to Tokyo. Only Thatcher and Mitterrand left their spouses home.

Earlier in the day, Anna Craxi, Hannelore Kohl, Mila Mulroney and Nancy Reagan got together for the first time since arriving in Tokyo.

They attended a Japanese tea ceremony with Tsutako Nakasone at the Urasenke tea headquarters, where people study the highly ritualistic tradition.

Leaving their shoes at the door, the women sat on stools at low tables facing Urasenke Grand Tea Master XV Soshitsu Sen.

Drinking a tea called "green clouds," Anna Craxi delighted the grand master by asking for a second helping. And Tsutako Nakasone, also following tradition, asked permission to admire the bowl.

The prescribed ritual is to turn the tea bowl clockwise two times so that one drinks from the back of the bowl. Mrs. Reagan failed to do so, but none of her companions seemed to notice.

In the morning, she visited the Nagata-Cho Elementary School, where she joined hands with the children and swayed with them as she sang "You are my sunshine, my only sunshine."

Besides a mural of Washington, D.C., and a pack of letters sent by students at their sister school, Martin Luther King Elementary School, Mrs. Reagan gave them a stack of Statue of Liberty T-shirts and a limited-edition two-foot model of the Statue of Liberty, with a letter of authenticity. It was signed by Lee Iacocca.