For Ronald Reagan, the first act of his presidential venture on the world stage proved a difficult and sometimes troubling experience that narrowly avoided becoming a disaster.

The good news for Reagan, as the Versailles summit concluded tonight, was that the key symbolic events of his 10-day European trip--Monday's meeting with the pope, Tuesday's address to the British Parliament and Friday's visit to the Berlin Wall--still lie ahead.

But on the negative side, the confusing signals given by U.S. officials on the Falklands war, particularly at Friday's U.N. vote, have provoked uncertainty and even anger among the British. Furthermore, the new outbreak of fighting in Lebanon, which demonstrated in a grim and dramatic way the difficulties of maintaining U.S. restraint over Israel, "cast a pall" over the final hours of the summit, according to Treasury Secretary Donald T. Regan.

Regan, who substituted for the president Saturday at the summit's trade discussions while Reagan was making a radio address to the United States, and Secretary of State Alexander M. Haig Jr. were the most visible representatives of the administration. Along with a supporting cast of other administration officials, Regan and Haig did most of the talking for Reagan on Lebanon and other issues.

Throughout the weekend, except when he appeared on French television during ceremonial occasions, the president remained unseen by most of the large entourage of reporters traveling with him to Europe.

After today's concluding session, for instance, Reagan was the only one of the seven world leaders participating in the summit who remained in isolation. While the heads of government of France, Britain, West German, Italy, Canada and Japan were answering questions from reporters, Reagan was resting in preparation for tonight's dinner after a short conference with Philip C. Habib, his special envoy to the Middle East. The president dispatched the veteran diplomat to Israel with the hope that Habib once more can achieve a cease-fire in war-torn Lebanon.

White House officials confirmed that Reagan had not planned to meet with reporters and discuss the summit even if the Lebanon issue had not arisen. Instead, the president issued a 1 1/2 page statement reaffirming support for the "spirit of partnership" that he described as the animating force of the Western alliance.

"In times of economic stress, it is always tempting to seek simple solutions at the expense of others," the statement said. "At Versailles, we resisted this temptation. Instead, we concentrated on ways and means to strengthen our economic performance individually and collectively."

The official American view of the summit's accomplishments--like those of the representatives of other nations here--was positive. Most administration spokesmen set great store by a vaguely worded passage of the summit communique agreeing that the allies would attempt to "limit" trade with the Soviet Union and its allies. No one was willing even to guess what those limits might be.

Because of the private nature of the summit meetings and Reagan's relative invisibility outside of them, it was difficult to assess the personal role played by the president in the deliberations that led to today's generally worded communique.

Haig maintained that Reagan was "an extremely significant factor" in the discussions. Other American officials said that the president insisted, at a strategic moment in the deliberations, on resolving the East-West trade issue before the participants moved on to the "North-South" issue, the phrase used to describe relations between industrialized and developing nations.

But no one could recall a specific Reagan comment on any issue that came up at the summit. One aide said that Reagan expressed sympathy during this private meeting with Habib for Shlomo Argov, the Israeli ambassador to Britain, who was critically wounded in an assassination attempt last Thursday.

The official American statements today--issued by Haig and deputy White House press secretary Larry Speakes--focused not on the London attack but on the violent response from Israel that it produced. The White House appealed to Israel to withdraw its forces, and the statement issued by French President Francois Mitterrand on behalf of the seven summit participants expressed "shock" over the news from Lebanon.

"We are deeply moved by the loss of human lives, the suffering and the destruction," Mitterrand said. "We think that this new cycle of violence, if it were to continue, could have disastrous consequences for the whole area."

The Lebanese fighting introduced to Reagan's European odyssey what one White House official called "a new element that will have consequences throughout the trip."

The other fighting, in the Falklands, also continued to have an impact.

Mitterrand today strongly expressed the summit nations' support for Britain, which he described as the victim of Argentine aggression. British Prime Minister Margaret Thatcher said at a subsequent press conference that she was especially pleased with the statement and the continuing "staunch" support of the United States for Britain in its war with Argentina.

But British newspapers jumped on the U.S foul-up that occurred Friday when Haig instructed United Nations Ambassador Jeane J. Kirkpatrick to abstain on a Panamanian-Spanish resolution on the Falklands that was vetoed in the Security Council by the British. Haig's order, relayed through Washington, arrived a few minutes late, and Kirkpatrick voted "no" on the basis of previous instructions.

Today, the British press weighed in. "Double Cross," proclaimed the tabloid Daily Mail, adding, "Buenos Aires Cheers as Haig Stabs Thatcher in the Back." An overline said, "U.S. Vote Chills the Reagan Visit."

The Daily Telegraph headlined, "Thatcher Dismay at U.S. About Turn." An editorial in the same newspaper referred to "a dreadful display of muddle and indecision." The Sunday Times headlined, "Mrs. Thatcher's Angry Silence," with a subhead that declared, "Reagan Vague on U.N. Mix-Up."

This response, coming on the eve of Reagan's visit to London, caused concern at the White House. Haig, who had tried to dismiss the affair as a "nuance issue" on Saturday, addressed the issue seriously today and said that U.S. abstention on the U.N. resolution "should in no way be interpreted as any lessening of the United States' support for the principle involved, which Great Britain is upholding."

The hope in the White House is that the Haig statement--and Thatcher's acceptance of U.S. good intentions--will represent the end of a controversy that is likely to be crowded out by the events ahead.

On Monday, these events include Reagan's historic meeting with the pope--with remarks timed to coincide with U.S. morning television shows--plus a meeting with Italian Premier Giovanni Spadolini and a flight to London, where the Reagans will be guests at a private dinner at Windsor Castle hosted by Queen Elizabeth II.

The papal meeting and the arrival ceremony with the queen are frankly "media events" of the kind at which Reagan usually excels.

The president's friendly manner and command presence stand him in good stead at ceremonial affairs, as they did at a series of dinners and receptions in Paris and Versailles. The European questions about Reagan reflect larger doubts about his mastery of complex issues--essentially the same concerns that domestic critics have raised about him in the past.

These were not concerns that Reagan set to rest in the first phase of a European trip that had been intended to demonstrate his capability of international leadership.