There is "considerable evidence" that the Soviet Union has violated the unratified SALT II strategic arms treaty and, if that is the case, "there's no need for us to continue" to honor it, President Reagan said yesterday.

Reagan, speaking in Lisbon before his return here from Europe yesterday, said he has not decided whether the United States will deliberately exceed SALT II limits on strategic weapons later this year when a new nuclear ballistic-missile submarine, the USS Alaska, begins sea trials.

At that time, the president must decide to exceed the SALT II limits on missile launchers or continue adherence to them by retiring other submarines. A decision is expected this fall.

In a 20-minute news conference in the gardens of Queluz Palace, the Portuguese official guest residence, Reagan also described his 10-day European visit as a "mission accomplished" and a "long, historic and thoroughly worthwhile trip" that produced "lasting achievements."

Later, in a statement on returning to the White House, Reagan said of demonstrations in Europe against his visit, "Everytime I noticed who was demonstrating, I felt reassured we were saying and doing things right."

In Lisbon, he reiterated a desire to meet with Soviet leader Mikhail Gorbachev while dismissing Gorbachev's harsh criticism in recent days.

Reagan acknowledged that other nations have refused to join his economic sanctions against Nicaragua.

On the strategic arms treaty, Reagan was asked whether he agrees with Assistant Secretary of Defense Richard N. Perle, who said in congressional testimony that the Soviets have violated the pact and that the United States should allow it to lapse.

Perle and other administration officials have cited Soviet construction of two new mobile missiles, the SSX24 and SSX25, as evidence that Moscow is no longer abiding by the treaty. Signed in 1979 but never ratified by the U.S. Senate, the pact limits each nation to developing one new missile.

The Reagan administration has charged that the Soviets also violated the treaty by using secret coding of data from test missiles, making it more difficult for the United States to monitor their performance.

Speaking of the treaty, Reagan told reporters:

"We have tried on what seemed to be a verbal agreement between ourselves and the Soviet Union for some time that, even though we had not ratified that treaty, it had been signed by the negotiators, that we would both seek to abide by the terms.

"There's considerable evidence now that that has been rather one-sided.

"And if it has been, then there's no need for us to continue.

"But whether we do or not, that's a decision to be made down the road. Actually, we have not come to a point in which we, in any way, in our own buildup are violating or going beyond the terms of that treaty.

"It is possible with regard to system of weapons that we might come to such a point. And we'll make that decision then. And if we do, we'll do it openly and we will do it with a full knowledge of the Soviet Union."

Reagan also said Perle was not speaking for the administration. "First of all, you know, in the country of ours, everyone's got a right to express an opinion and he was doing no more than that," the president said.

Perle's remarks came last week on the eve of Reagan's major address to the European Parliament. In it, he promised to seek better relations with the Soviets.

On a possible meeting with Gorbachev when the United Nations session opens in September, Reagan said "we have no confirmation" that Gorbachev will be there. "The word 'probable' is about the best way to describe it," he said.

Reagan said he had encountered a "philosophical difference" with other nations on the Nicaragua sanctions. "There are a number of people and certainly a number of governments who just don't believe in that as a legitimate weapon," he said.

He added that "many of our own people in the Congress" suggested use of sanctions. He said the United States had maintained economic relations with Nicaragua earlier to "refute their charges that we were somehow threatening them with aggression, and if you'll remember, there was a time when Nicaraguan President Daniel Ortega had us, every other week, landing the Marines in Nicaragua and we never had any intention to do such a thing."

After the recent congressional impasse on aid to the anti-Sandinista contras, or counterrevolutionaries, Reagan said, he decided "that pressure is needed to bring them to the realization that they should restore the original goals of the revolution."

In a statement read to reporters, Reagan said he is satisfied with the outcome of his European visit.

He said his five days in West Germany had "strengthened U.S.-German relations," but he did not mention the controversy about his participation in the wreath-laying ceremony at a German military cemetery where Nazi SS soldiers are buried.

At the Bonn economic summit, Reagan said, "we moved closer" to launching a new round of global trade talks. "All but one" nation, France, agreed that it should begin next year, he said.

At the European Parliament, where he was heckled by leftists, Reagan said he had laid out a "sensible framework for improved Soviet relations."