STOCKHOLM, JUNE 9 -- Swedish officialdom rolled out the red carpet for Nancy Reagan today as she called upon the queen, the prime minister's wife and the widow of his predecessor, who for two decades before his assassination in 1986 had been a frequent and severe critic of U.S. foreign policy in Vietnam and later Central America.

Meanwhile, demonstrations against U.S. Central American policy continued to follow Mrs. Reagan for a second day. Asked her reaction, the first lady said, "I really don't think much about it.

"I'm here to talk about drugs," she said.

For the Swedes, the first lady's visit, her first here, has been seen as the latest in White House efforts to assure Sweden that despite sometimes sharp differences in the past, it remains one of Washington's good friends.

This spring, the United States added Sweden to its list of privileged trading partners by exempting it from export controls on the licensing of high-tech products. In May, announcements were made that Mrs. Reagan would visit Stockholm and that in September Prime Minister Ingvar Carlsson would make an official visit to Washington, the first by a Swedish prime minister in 25 years.

Today, Mrs. Reagan's visit to Rosenbad, the offices of the prime minister, included a photo opportunity with Carlsson and his wife Ingrid and a session with former prime minister Olof Palme's widow Lisbet.

Later, Mrs. Reagan had lunch with Queen Silvia at Drottningholm Palace, where other guests were Prince Bertil, uncle of King Carl Gustaf XVI; ; Princess Liliane; Princess Christina and her husband Tord Magnuson. Then she and the queen attended an antidrug forum hosted by Swedish Lions Clubs in cooperation with several nongovernment organizations.

Outside, mounted police outnumbered about 50 protesters and later detained three who tried to burn an American flag outside U.S. Ambassador Gregory Newell's residence before Mrs. Reagan arrived there for dinner.

Though Mrs. Reagan did not comment on the demonstrations, Ulla Wachtmeister, wife of Swedish Ambassador to the United States Wilhelm Wachtmeister, discounted them.

"Oh, that's everywhere. It's always those crazy youngsters. They want to be on front pages everywhere," said Mrs. Wachtmeister, who flew here especially for Mrs. Reagan's visit.

In a speech at the antidrug forum, Mrs. Reagan said she has been impressed with the programs she has seen here and said, "there's no doubt that we will win the battle against drugs with these continued efforts, along with international cooperation."

Recalling a letter she once received from a young girl whose brother overdosed on drugs, the first lady seemed to choke up when she described how the girl had written "help" in large letters at the bottom of the page.

"Each of us has the moral obligation to provide that help," she said. "We must do more than simply recognize the problem -- we must take a personal stand against drugs. If we don't we are accepting a practice that kills."

Earlier, at the Moringen Women's Treatment Center, a rehabilitation program for pregnant drug addicts, she met with three mothers, all of whom had their babies with them.

Holding 6-month-old Robert, Mrs. Reagan asked his mother whether she gave up drugs upon becoming pregnant. Marie Hynynen, one of two women currently in residence, said she had and never again wanted drugs.

"Well," Mrs. Reagan jokingly remarked, "the answer is for everyone to have a child, obviously."

Former resident Gith Johnsson, mother of two children, asked Mrs. Reagan's "personal motivation" in involving herself in the drug problem.

"Well, because I think it involves the future of all of our countries," the first lady replied. "We're dealing with the next generation who we're going to hand the world over to."

Accompanying the first lady was Dr. Donald Ian Macdonald, special assistant to the president and director of White House drug abuse policy, who told reporters that Mrs. Reagan's remark that everyone should have a baby was not "flip."

"It's sort of a good thought because what pregnancy does for a number of women is give them motivation," said Macdonald, a pediatrician.

Mrs. Reagan will return to Venice Wednesday to rejoin President Reagan as he winds up talks with other leaders at the seven-nation economic summit.