In a frenzied final day filled with the unexpected, Soviet leader Mikhail Gorbachev reached out to meet some average Americans, then boarded his huge Aeroflot jet to leave the city that he has captivated for the past three days.

Gorbachev seemed to be everywhere: on the sidewalk at Connecticut Avenue and L Street under twinkling Christmas trees, grasping the hands of thrilled passersby; back at his embassy, charming American youngsters; holding forth to a worldwide press assembled at the Soviet compound on Wisconsin Avenue NW; and then waving from the steps of his rain-slicked plane at Andrews Air Force Base.

It was the first time in his three-day visit that he had gone beyond the politicians and actors and insiders to make a gesture to those who had been waiting on Washington sidewalks to catch his eye. Americans seemed enthralled by his touch.

The city's excitement was expressed by a 10-year old boy who waited with a crowd in the rain near the Soviet compound on Wisconsin Avenue NW for Gorbachev's arrival to meet with reporters. As his huge black ZIL limousine drove past, Gorbachev smiled and waved.

"I thought it was awesome," said schoolboy Haven Pell. "I'm going to write a composition about it."

The frantic pace of Gorbachev's final day in Washington seemed in keeping with the super-charged atmosphere his visit had created. In his brief stay and limited forays around the city, Gorbachev got a taste of America's diversity. He saw beaming office-workers and waving youngsters -- while within three blocks of his embassy he and his government were vilified in several different languages by groups as diverse as Ukrainians and Ethiopians.

Even as the Soviet motorcade was departing the city last night, contractors hired by the Secret Service were dismantling the huge concrete barriers near his embassy that were the most visible signs of the intense security surrounding the visit. The miles of yellow police ribbon and battalions of officers around town seemed to evaporate as quickly as they had appeared in the predawn hours Monday. Law enforcement personnel breathed a sigh of relief, as they readied the city's downtown streets for their first normal rush hour in days.

The last send-off came at about 8:50 p.m. as a steady rain pelted the parking ramp at Andrews. Gorbachev and his wife Raisa walked up the soggy red carpet, turned for a final wave and nod, and disappeared into the Soviet airplane.

As the plane lifted off at 9 p.m., Air Force officers immediately prepared for what they called a "wheels-up pizza party" in a nearby tent.

Americans marked his departure in different ways.

A class of seventh graders in Owensboro, Ky., sent 70 pounds of children's blue jeans to Gorbachev, destined, they hoped, for children in the Soviet Union. "It was our way of getting in on the peace talks," said Lisa Newcom, the teacher. "We were sending a gesture of freedom and good will and peace on the seventh-grade level. We thought it was something we could do."

The big brown cardboard box ended up at the Vista Hotel, near 15th and M streets NW, along with a load of other cardboard boxes addressed to the General Secretary of the Communist Party of the U.S.S.R. There was a box marked "Girl Scout Peanut Butter Sandwich" from Lafayette, La., and another box, marked "Fragile," containing a colored print titled "Doves of Peace" from Greenwich, Conn. The artist, Lee James Pantas, said he wrapped it "on impulse" and sent it off "to this man of peace."

In Washington, 15 students, who also looked to Gorbachev as a symbol of peace, had worked for days to meet him. And when they did, yesterday at the Soviet Embassy, they found it "absolutely incredible," as 17-year-old Heather Iliff later said. The Severna Park, Md., high school student said Gorbachev was "an incredible, vivacious, friendly man."

Classmate Lisa Fletcher, also 17, said that if any members of the group had negative preconceptions, such thoughts were "blown away."

The warm feeling apparently was mutual for the Soviet leader. At his press conference, he referred to his meeting with the young people, saying that they, unlike adults, are able to "quickly find a common tongue . . . . They are thinking about how we should live in this world," a phenomenon which he called "remarkable."

Adults too, perhaps jaded by the almost routine visits of foreign dignitaries in this town, have reveled in the summit all week. In Mel Krupin's restaurant, where celebrity-watching is almost on the menu, a visit by a small delegation of Soviets had diners on their feet, toasting and offering to buy drinks for the visitors. At the Market Inn, a Capitol Hill haunt, the management wanted the Soviets to feel right at home. They placed Russian-language signs for "Men" and "Ladies" on the restrooms.

Gorbachev's constant comings and goings on 16th Street NW spawned a set of Gorby groupies. Jane Hatheway watched through binoculars all week from her desk at the Independent Petroleum Association until her boss told her someone might misunderstand her intentions. She came to the corner as often as she could to catch sight of the motorcade, and when she was disappointed Wednesday at a fleeting glimpse, she joked, "Maybe tomorrow I can throw my body on top of his limousine."

Workers at the Madison Hotel at 15th and M streets NW, home to more than 200 Soviet diplomats for the past days, found the Soviets delightful.

"I think the people are lovely," said a woman who checks coats near one of the hotel's restaurants. "They're very gracious. They're very polite . . . . I find them like us, very ordinary people."

Even U.S. Chief of Protocol Selwa Roosevelt was struck by Washington's embrace of the visitor. "Washington is fairly blase," she said. "We have celebrities here every week, but this time so many are waiting for a glimpse."

Yesterday's day of communion followed a night marred by at least one unpleasant incident, still unresolved. There was a telephone threat on Gorbachev's life made to police headquarters Wednesday night. Police traced the call to a downtown phone booth, where witnesses helped police locate a man, whom they later arrested on a charge of escaping from a halfway house.

The Secret Service and FBI are continuing to investigate the incident, according to a D.C. police official. Both federal agencies declined to comment.

Officials involved in the massive security effort surrounding Gorbachev's visit said it was too early even to estimate the cost.

Hundreds of D.C. police officers have been working 12-hour shifts since Saturday, manning security barricades, escorting motorcades and handling demonstrations. They have been joined by U.S. Park Police, who also escorted motorcades and had primary responsibility for keeping order at demonstrations in Lafayette Park and on the Mall. The Secret Service, which is in charge of the entire operation, has provided round-the-clock protection to Gorbachev, his wife and several other top-ranking members of his entourage.

"It's a costly event," Assistant D.C. Police Chief Isaac Fulwood said. "But we don't even know how much yet . . . . It's expensive."

Though it is difficult to compare events, the 34-hour visit of Pope John Paul II in 1979 ran up a $950,000 overtime bill for D.C. police and U.S. Park Police. The largest chunk of that was for the city police overtime, about $800,000.

John White, spokesman for Mayor Marion Barry, said the State Department will be billed for the costs of the Gorbachev visit because the operation was mounted at the request of the federal government.

During the past six days, law enforcement agencies have had to contend with more than a dozen demonstrations, most of them decrying some aspect of Soviet policy -- and almost all of them peaceful. The rallies' organizers used Gorbachev's last day in the United States to reflect on their efforts.

Organizers of the large rally Sunday on behalf of Soviet Jews were elated at their turnout, and said its size -- more than 200,000 participants, or the equivalent of about one in 30 of the nation's Jews -- was "an extraordinary mandate," said Pamela Cohen, president of the Union of Councils for Soviet Jews.

Her group is following up on Sunday's rally with a new campaign to press for more Jewish emigration from the Soviet Union and increased human rights. The campaign will organize boycotts and use other tactics against companies and banks that do large amounts of business with the Soviets. Among the targets, she said, could be companies owned by Armand Hammer, who has done business with the Soviets for years, and Pepsico Inc.

Myron Wasylyk, Washington representative of the Ukrainian Congress Committee of America, said that his organization's rally, which brought 1,500 people to Lafayette Park Monday to protest Soviet treatment of Ukrainians, was carried live on Voice of America radio to millions of Ukrainians. "Any voice of support from the White House or the Ukrainian community here for the Ukrainians there is heartening to them," said Wasylyk, "We made the point."

Yesterday, Fulwood made a private point, with a foreign colleague. He exchanged color pictures and badges with the head of the KGB detail guarding Gorbachev. "His parting words to me were that he wished the American people well and that he was my friend."

Staff writers Victoria Churchville, Lynne Duke, Keith Harriston, David Hilzenrath, Sari Horwitz, Eugene Meyer, Tracey A. Reeves, Carlos Sanchez, Molly Sinclair, Laura Sessions Stepp, Martin Weil, Linda Wheeler and Jeffrey Yorke contributed to this report.