Nancy Reagan, expanding her role as a catalyst in mobilizing parents against the growing international problem of drug abuse, met with a Portuguese parents group today.

"I am a big, big believer in parents groups," she said, with Manuela Eanes, Portugal's first lady, seated at her side in a pink frescoed drawing room at Queluz Palace, where the Reagans are state house guests.

"Government has its place in trying to interdict drugs coming into the country, in punishing pushers," Mrs. Reagan continued, "but in the final analysis, it is the parents groups who are going to make a difference."

Manuela Eanes organized the get-together after she returned from the first ladies' drug conference in Washington. She said she felt that Mrs. Reagan's presence was "an excellent way to call attention to a problem that is destroying the lives of so many of our young people."

The Portuguese government estimates that about 80,000 Portuguese youth out of a population of 2 million between the ages of 12 and 25 are drug-dependent. With Portugal becoming a major distribution center in Europe for drugs from South America, Africa and the Middle East, heroin use rose from 14 percent to 83 percent among those who underwent drug rehabilitation in 1983-84.

Among the parents representing the Association for the Prevention of Toxic Substances was a microbiologist who said her son's drug addiction was "the hardest problem I ever had in my life. I felt guilty and thought I had been overambitious, and for leaving my marriage of 20 years."

"Did you know your son was on drugs?" Mrs. Reagan asked Laura Vasconcelos, who was on the verge of tears while telling her story.

"No," Vasconcelos said. "I thought it was just a phase he was going through. At the time, he was undergoing psychiatric treatment and I thought he was in a state of trauma and depression."

She said her son later underwent successful drug rehabilitation treatment and that "the two of us entered into a contract in which rules of behavior were laid down. We were helped by another mother and a friend offered him a job."

Mrs. Reagan urged parents to become knowledgeable about drugs so they can recognize the symptoms and not just think they are "a stage their children are going through. It's easy to be confused if you're not knowledgeable."

Calling Mrs. Reagan's interest "a great incentive," architect Antonio Carvalho, president of the nine-month-old group, said its priority "is to educate and inform the public of the consequences of drug abuse."

Maria Amalia Serrao Fialho, a biologist who was among the original 20 parents forming the group, told of feeling "ashamed, humiliated, despairing and guilty" at first.

"We realized that families were in a state of crisis and had a responsibility," said Fialho. "But we also realized there was a state of crisis in our schools, society and the world in general and that families didn't have the answers."

Nancy Reagan brought more than a few answers and words of encouragement to the group. "I'm encouraging you to the point that I brought you some money," she said, handing Carvalho a $5,000 check from the National Federation of Parents for a Drug Free Youth. It is the same Maryland-based group that sent a $5,000 check to a West German parents group in Bonn last week.

From here, Mrs. Reagan, with President Reagan, went on to a luncheon given by Prime Minister Mario Soares at the 15th-century National Palace of Sintra. Toasting his guest with vintage ferreirinha porto after a meal of sole, chicken and chocolate mousse, Soares noted that Reagan's visit was the first by an American president, "which proves how the former Portuguese dictatorship isolated our country for many years."

Replied Reagan: "With courage and tenacity you cast off the chains of a dictatorship, defeated those who would have subverted your cause and have built a government based on the popular vote and a respect for human rights."

Of the palace, with its Gothic, Manueline and Islamic architecture, and the centuries-old village in the Serra da Arra'bida mountains above Lisbon that Lord Byron called "the glorious Eden," Reagan also said, "We can now sense what he felt when he penned those words."

The 110 invited guests included former Portuguese presidents and prime ministers, business and political leaders and candidates for the November presidential elections. Among the Americans present were Secretary of State George Shultz, White House chief of staff Donald Regan and national security adviser Robert McFarlane. For outgoing deputy chief of staff Michael K. Deaver, overseeing his swan song after four years of presidential trip-planning, the luncheon setting seemed particularly appropriate. It was held in the palace's Swan Room.

This evening, the president and first lady, holding hands, came to a reception for the press at Queluz Palace given by U.S. Ambassador Allen Holmes. The Reagans appeared to be very fit and in a good mood. The president joked that the press was very tired but he didn't understand why.

Nancy Reagan wore what appeared to be a pink Galanos gown with an extravagant hand-beaded bodice featuring a V-neck and long sleeves. They were on their way to the state dinner that President Antonio Ramalho Eanes gave in their honor.

The party was off the record, but was an opportunity for top staff members, like Deaver and Regan, to trade barbed quips with reporters about coverage of the president's trip. Since it was Deaver's last day on the White House staff -- he's off the payroll tomorrow -- he appeared to be feeling a bit sentimental.

Reagan, who failed to recognize Soares at the Wednesday night arrival ceremony, often confuses White House reporters. He has mistaken, for instance, the Los Angeles Times' George Skelton for The Washington Post's Lou Cannon. Tonight, however, Time magazine's Larry Barrett was called "Lou" and CBS' Lesley Stahl was called "Andrea." NBC's Andrea Mitchell had just finished chatting with Reagan.