Metal detectors have long been in vogue at Washington parties for famous people. But last night, at a reception for Lebanese President Amin Gemayel, you could get all dressed up, chatter excitedly in the receiving line as you rounded the corner toward the Madison Hotel's ballroom and then, with arms held out at your sides -- be frisked.

"We had to take precautions," said Lebanese Ambassador Khalil Itani, "after what happened to his brother."

President-elect Bashir Gemayel was assassinated last month in Lebanon; his 40-year-old brother, the new president, ended his visit to Washington yesterday with a reception that, even by local standards, was a jungle. Almost 1,000 people shoved and oomphed to check out a man who, because of his power, youth and presence (he broke into tears after being elected last month) is a natural for political celebrity watchers.

"Excellent, excellent," Gemayel said of his meetings yesterday with Ronald Reagan. Gemayel had asked for an expanded multinational force in Lebanon and, while Reagan didn't commit himself, a member of Gemayel's entourage said last night that "we detected he was in favor."

"We were very reassured," said the entourage member, Ghassan Tueni, the former Lebanese ambassador to the United Nations. "The president said he was considering that sympathetically."

Many of the guests were Lebanese-American executives, invited from all parts of the country. The Lebanese Embassy, which threw the party, wants them to invest in the $10 billion reconstruction effort in Lebanon.

"If I get the right opportunity, I will," said Gabriel Phillips, senior vice president of a trade association for domestic airlines. "I might," said Joseph Barakat, whose company manufactures military communications systems.

No one from the top White House staff was spotted at the party, but Secretary of State George P. Shultz came and stayed longer than Cabinet members usually do at embassy receptions. "Thank you for everything, thank you for everything, thank you for everything," Gemayel said to Shultz in the receiving line. "Everything was excellent."

"A very impressive man," Shultz said afterward. "You know, I don't like being interviewed at parties." And that was it.

Although the Lebanese Embassy gave the party, the Lebanese Information Center and the American Lebanese League helped put together the guest list. They say they invited 750 to the party that was rapidly assembled into an unorganized casserole.

"We couldn't make up our minds -- was it possible to put something like this together or not?" said Ghassan Hashim, a spokesman for the Lebanese Information Center. "Finally, it was put together on Thursday."

Gemayel himself left the party a half-hour early, excusing himself to catch a plane for Paris. Flushed and beaming, he had kissed hundreds and hundreds of cheeks, of both men and women, some as many as three times. The Lebanese press attache' watched him go, then shrugged. "He was tired," he said.