VENICE, JUNE 3 -- President Reagan arrived here today on a nine-day trip to Europe, pledging a renewed commitment to the western alliance in an all-out effort to sustain "the longest period of general peace in this century and the greatest prosperity in the history of man."

On Air Force One en route to Venice, national security adviser Frank Carlucci told reporters that the Reagan administration had "completed its review of what suggestions" it should make to the allies in assisting the United States to protect oil tankers in the Persian Gulf.

Carlucci noted that West Germany and Japan have constitutional restrictions on their military roles but that France and Britain already were providing naval protection for their own vessels in the gulf.

"What is it that we want those ships to do?" Carlucci said. "It's as much a presence and coordination matter as much as anything else."

State Department officials said in congressional testimony last week that U.S. credibility in the Middle East had been damaged by the Iran arms deal. But Carlucci disputed this, saying, "If we had totally lost our credibility, the Kuwaitis wouldn't be asking us to reflag vessels."

Reagan has vowed repeatedly to keep oil flowing through the gulf despite Iranian warnings that tankers and the ships that shield them would be considered military targets in the Iran-Iraq war, which has lasted nearly seven years.

Carlucci said arms control would be a major topic when the seven leaders of the industrialized democracies meet here next week. He said they were "close to a common position" on an agreement to eliminate intermediate-range missiles in Europe.

Carlucci also said the administration believes that "we need to press ahead vigorously" toward an agreement on strategic nuclear weapons.

Reagan will relax at a 17th century villa 11 miles north of here for two days before going to Rome and a meeting with Pope John Paul II. Beginning Monday he will attend the 13th annual economic summit, which will last three days.

Reagan will conclude his European trip, which could be the last of his presidency, with a speech at the Brandenburg Gate in West Berlin on June 12 and a meeting with West German Chancellor Helmut Kohl before returning to Washington that day.

"On this trip, I will commemorate the 40th anniversary of the Marshall Plan," Reagan said in a departure ceremony on the south lawn of the White House before flying here. "Forty years ago the United States said that if Europe were ever to see an end to the specter of war that had haunted that great continent over the centuries, all of its peoples would have to know freedom, democracy and justice. And so we extended both to allies and former enemies a helping hand, a hand of compassion, a hand of hope."

Reagan went on to say that after the Soviet Union declined to participate in the Marshall Plan, the United States helped rebuild Europe.

"At this economic summit, I will look around the table and see -- thanks in part to the generosity and wisdom of our nation over the past 40 years -- not the leaders of broken, desperate and despotic nations, but the leaders of strong and stable democracies, countries that today are our partners for peace on the world stage," he said.

However, a number of these world figures -- like Reagan himself -- appear to be in the twilight of their leadership.

Japanese Prime Minister Yasuhiro Nakasone is expected to step down in October. Canadian Prime Minister Brian Mulroney, a close ally of Reagan, is at the nadir of his popularity after a year of scandals.

Here in Italy, where elections will be held June 14, longtime political figure Amintore Fanfani is heading a caretaker government. Fanfani greeted Reagan when he arrived here tonight at Marco Polo Airport and will represent Italy at the summit talks.

British Prime Minister Margaret Thatcher, Reagan's firmest ally among the alliance partners, will be here for only one day of the summit because of the British elections, which will be held June 11. Thatcher is considered the strongest of the summit leaders, and her Conservative Party is favored to win for a third consecutive time.

West Germany, where Kohl's Christian Democratic Party recently lost two state elections, and France are the other participants.

"At Venice, we will talk about how to improve East-West relations," Reagan said in a departure statement to students of James Madison High School in Fairfax County. "We will discuss arms reduction, human rights problems, regional conflicts and bilateral cooperation. Our discussion in Venice will help strengthen western solidarity, which is indispensable to progress on issues of contention between East and West."