Mikhail Gorbachev's limousine screeched to a halt at noon yesterday, and in an instant there was a hysterical buzzing around the car. Horrified security agents yelled to the crowd, "Keep your hands out of your pockets!" People on the sidewalks watching the Soviet leader's motorcade to the White House thought something had gone terribly wrong.

Not until they saw Gorbachev's unmistakable pate bob up in the crowd did everyone realize that they were witnessing not a disaster but a classic, American-style whistle stop at one of Washington's busiest intersections: Connecticut Avenue and L Street NW.

With an interpreter trailing him, Gorbachev stepped up to the first woman he saw, shook her hand and said, "Hello, I'm glad to be in America. I'm glad to be friends with all of you."

The cars in the motorcade ahead of Gorbachev's boxy ZIL limousine didn't seem to realize at first what was happening. When they did, the drivers slammed on the brakes and backed up at a terrific speed. The District police made furious U-turns on their motorcycles, and flatfoots beat it down the street with their hands on their holsters.

"It looked like a scene out of the Keystone Kops," said Leslie Kobylinski, who was on a break from her job at the U.S. Chamber of Commerce.

"He's out!" one police officer shouted. "He's shaking hands!"

Surrounded by nervous security agents in dark suits, the general secretary of the Communist Party of the U.S.S.R. spent two minutes at the curb in front of 1050 Connecticut Ave. waving, schmoozing and shaking hands with a half-dozen people. He worked the crowd as if he were the front-runner in an upcoming District of Columbia primary.

Calm amid the giddiness, Gorbachev told Lise Murphy, a passer-by, "My people are pushing me very hard to come to a better understanding of the American people."

It took a few moments before everyone joined in the spirit of the moment. "Some people seemed hesitant to shake hands with him," Murphy said. "They said, 'May I shake your hand?' and he reached right over and grabbed their hand. He seemed like a presidential candidate."

When restaurateur Duke Zeibert saw what was happening on the street below, he bolted to the balcony of his popular eatery barking, "Come on up and have lunch! We have borsch!"

And Howard McNamara, a bartender at Zeibert's, managed to snap a roll of photographs of Gorbachev being mobbed by the lunchtime crowd.

George Bush came out of Gorbachev's car looking very vice presidential as he stood nearby on the street and shook hands with a much smaller crowd. Soviet Foreign Minister Eduard Shevardnadze stood to the side smiling as if Gorbachev's impromptu stop in the middle of Washington was a daily event.

American security officials dread spontaneity, but that has been one of Gorbachev's political trademarks since he came to power nearly three years ago. After a month in office, he worked the crowd at a Leningrad shipyard, and last year on the way to an airport in New Delhi he called for his car to stop so he could meet with a group of Indians by the highway.

By the time he made his stop yesterday, Gorbachev was already an hour late for an appointment at the White House with President Reagan.

Connecticut Avenue had been cleared for Gorbachev. The side streets were shut off by barriers and police. Yards and yards of clear pavement. Squad cars everywhere. People standing on the street barely took notice of the helicopters whipping their blades overhead. Washingtonians have seen motorcades before.

Then the long string of cars and vans and motorcycles came snaking down the street.

Suddenly, Gorbachev's limo put down nasty screech marks. The car behind his almost slammed into its rear fender. In front of the Soviet Embassy on 16th Street NW, a police officer's radio crackled with an ominous message: "We have a traffic problem at Connecticut and L."

"I thought, 'My God, something's happened to him!' " said April Grierson, a phone company employe who was walking to lunch.

"I was sure something bad had happened," said Betsy Caine. "I was frightened."

Twenty or 30 security officials jumped out of various cars. "They were everywhere," Caine said. Agents leaped on a row of newspaper vending machines and helped form a protective cocoon around Gorbachev. They started looking to and fro as if they were watching a hysterical tennis match.

A Secret Service spokesman later said Gorbachev's decision to stop "was a little unusual. All we do is plan their security. If they want to walk or stop, that becomes part of the game plan."

Crowds quickly clogged every corner of the intersection.

Gorbachev leaned over the New York Post box -- "NEW NAZI ATROCITIES BARED!" -- and won the crowd. People flashed him the peace sign and yelled, "Welcome!"

"I was born and raised in D.C. and I've never even seen any U.S. president live before," said Pat Simonds. Gorbachev, said Barrie Kessler, a computer consultant, "is a great PR man."

At Duke Zeibert's, waiters and customers rushed to the balcony. Gorbachev looked up at the expense-account crowd and waved. The 40 people on the balcony and the crowd on the street burst into applause as television cameras rolled.

"Even from the balcony you could sense the charisma of the man," said Raleigh Schein, director of advertising at The Washington Post. "It was almost like a parade or a celebration. There was a world leader out shaking hands and you kind of felt the world was going to be okay. None of us wanted to let go of the moment. It was such a warm moment, of love. I'm a cynic, but I got chills."

"For about a minute there, I felt like a part of history," said John Williams, 24, executive assistant for the Council for Court Excellence. "As a Republican, I've always had a basic distrust of the Soviets. But this! It won't make a drastic change, but it softened me up a little. He must have done this all on his own. It showed a basic trust of the American people."

Indeed, Gorbachev told Bush that he decided to stop the motorcade because people in the Soviet Union react "favorably" to interacting with their leaders.

"I was driving along with him in the back seat of his car and we were getting a warm reception from the American people," Bush told a group of reporters later in the day.

Gorbachev spoke to the crowd in Russian, and his interpreter kept pace. "We come from two great countries," Gorbachev said. "It was so good to come here."

As he was turning to go back to his car, the applause grew louder and Gorbachev could not resist lingering at the door. He and Bush stood together and waved, looking very much like a triumphant campaign ticket. As the car drove away, Gorbachev kept his palm pressed to the window in farewell.

By the time he made it to the White House, he was an hour and a half late. As the car rolled up in the White House driveway, the president was there to greet it.

"I thought you'd gone home," Reagan said to Gorbachev.

Gorbachev smiled. "I had a chat with a group of Americans who stopped our car."

Staff writers Henry Allen, Chuck Conconi, Jeffrey Frank, Sari Horwitz, Caryle Murphy and Linda Wheeler contributed to this report.