He was still wearing his tuxedo from the state dinner for President Reagan as he strolled into the Meridien Hotel here looking for his guests.

He was the prime minister of Portugal, but he came without ceremony or fanfare -- just a few aides and an invitation to join him in listening to fado, Portuguese folk singing, at a favorite restaurant.

It was nearly midnight and Reagan's last night in Portugal, and Mario Soares, 60, who has an easygoing air and an expansive style, was in the U.S. pressroom chatting with reporters and the Portuguese telephone operators.

Soon he was on the street as aides were arranging for cabs to take the Americans to the restaurant. As head of Portugal's government at a time of economic difficulty, Soares still enjoys sitting down informally with journalists for talk of politics, philosophy and his country and its culture.

The destination was Sr. Vinho, a casa de fado in Lisbon's Bairro Alto district.

He ordered a glass of water and folded his arms on the table, and then the questions began.

What did he think of the elegant state dinner hosted a few hours earlier for Reagan by Portuguese President Antonio Eanes? Playfully, he said it was "bad," and shook his head.

"Why?" demanded the incredulous American reporters.

"Because it was official," said Soares, who is clearly more comfortable in less offical settings. His home and garden in Lisbon are where journalists and constituents have learned to find him when they want to get his attention, an aide said.

For American reporters at the end of a grueling 10-day European journey with Reagan, marked by heavy security, gun-toting policemen and one violent street demonstration, the night out with Soares was a refreshing exception.

No one demanded press credentials, baggage searches or walks through metal detectors to talk to the leader of Portugal.

And Soares did his own talking without the coaching of ever-present ministers and aides who appeared with other heads of government during the entire European tour.

His aides had expressed amusement over the minute-by-minute planning that had characterized the Reagan visit. And they said Soares chafes at the kind of security protection that other leaders have come to expect. His aides tell of the time he came to Washington and the Secret Service wouldn't let him wander around the capital.

The invitation to hear fado came casually earlier in the day at a luncheon Soares hosted for the Reagans in the palace at Sintra, a mountain village near Lisbon. The reporters were invited to hear the haunting and sad folk singing that Soares loves and is to the Portuguese what flamenco is to the Spanish.

The Portuguese are so reverent about fado that late arrivals are not seated when the fadista is performing. As the reporters crowded into a tiny vestibule at Sr. Vinho, one asked the proprietor if the management would keep even the prime minister waiting until the song was over.

The question was answered a few minutes later when Soares arrived and was kept waiting.

Inside, he seated himself at the center of a long table with paper Portuguese flags, and reporters crowded around. Luis de Sousa, the press attache' at Portugal's Washington embassy, did the interpreting. Soares understands English but prefers to converse through an interpreter.

A lawyer and journalist, Soares was long active in dissident political activity during the Antonio Salazar dictatorship. A socialist, he was a major leader of the resistance to an attempted takeover by the communists after the Salazar regime fell. Jailed 13 times without trial, he left Portugal in 1969 for exile in Paris and Rome, returning five years later.

On this night, he was asked whether he thought Reagan a smart man.

"You have to be to become president of the United States," he said.

What did he think of the controversy surrounding Reagan's wreath-laying at the German military cemetery at Bitburg? It wasn't of great consequence, he said, because it seemed clear that Reagan was not paying homage to the Nazis.

Are there many Jews in Portugal?

"We are all Jews!" he responded, adding that it would be impossible to answer the question accurately.

Why would Soares, now Portugal's chief executive, want to run for the largely ceremonial office of president, as is widely rumored in Portugal?

"I am not a candidate," he protested. He then smiled broadly as one correspondent asked him, "If nominated will you run; if elected will you serve?"

After a few quizzical looks, Soares then got a lesson on the origins of the classic categorical statement, named after Civil War Gen. William Tecumseh Sherman, who made it.

The prime minister then said that "theoretically, strictly theoretically, and this is not my case," the reason a prime minister might run for president is that Portugal's decade-old democracy has yet to have a civilian in that post.

A hush fell over the restaurant as the first singer, whom Soares recognized as a prominent Lisbon judge, started to perform. Soares was soon singing to the strains of guitars and lyrics that recalled the traditional style of fado sung by the university students of Coimbra.

A somber moment came when the restaurant asked him to sign its guest book beneath the signature of Carlos Mota Pinto, his deputy prime minister and defense minister, who died May 7 of a heart attack.

He wrote a tribute to his late colleague in a careful hand and signed his name.

Still drinking only water, Soares bantered easily with reporters. Nicaragua is going the way of Cuba, he said, echoing Reagan. But he said Reagan's economic sanctions against Nicaragua were not the answer -- they would only make heroes out of the Sandinistas.

"You mean martyrs?" he was asked.

"Exactly," he said.

What about the Portuguese communists who walked out of the legislature before Reagan's address?

Well, he said, at least they were polite -- unlike the boisterous communist hecklers who greeted Reagan the day before at the European Parliament in Strasbourg, France.

It was 2 a.m. when the interpreter motioned to his watch. Soares got up gracefully, bid farewell and went home.