On the eve of the economic summit conference that opens here today, President Reagan and other top U.S. officials sought yesterday to mobilize world opinion against the Soviet Union for what the president called its "stubborn refusal" to provide a full account of the nuclear reactor accident at Chernobyl.

Reagan repeatedly dwelt on this theme and on a call for concerted action against "the scourge of terrorism" in meetings with Japanese and Italian leaders, a radio address and responses to Japanese journalists.

Although the drama of the Soviet nuclear accident continued to overshadow all other issues, several American and allied officials emphasized that forging greater unity on fighting terrorism remained the number one U.S. political goal here.

Denouncing Soviet secrecy in the aftermath of the Chernobyl accident, a senior U.S. official said that the incident had given the world "a different and less favorable picture" of Soviet leader Mikhail Gorbachev, who frequently has been portrayed in the West as a more modern and public relations conscious leader. It also made the case for "realism and verification" of agreements made with the Soviets, the official added.

"The Soviets' handling of this incident manifests a disregard for the legitimate concerns of people everywhere," Reagan said in his weekly radio speech. "The Soviets owe the world an explanation, a full accounting of what happened at Chernobyl and what is happening now is the least the world community has a right to expect."

Meanwhile, sources said Treasury Secretary James A. Baker III has brought to the economic summit meeting of the leaders of the seven industrialized democracies here a proposal for broader economic coordination among the industrialized nations.

While details were not disclosed, the sources said Baker was talking with finance ministers of other nations about a new mechanism for coordination that would not focus directly on exchange rates, but would seek joint action on broad economic issues such as inflation, growth and interest rates.

The sources said Baker had not yet achieved agreement on the plan, and may not get agreement at the summit. But they said Baker is seeking an extension of the agreement reached last September in New York among the nations to bring down gradually the value of the dollar.

Baker stepped up pressure on West Germany and Japan to stimulate their domestic economies. He warned that failure to do so could result in a recession in the United States.

The U.S. view is that substantially greater activity in the economies of its two principal trading partners is necessary to reduce the West German and Japanese trade surpluses. Baker said that a $100 billion U.S. trade deficit in 1987-88 would be "politically unsustainable."

Baker told reporters that the deficit is too large to be corrected by a further depreciation of the dollar alone, a move that tends to increase U.S. exports by making them less expensive.

"So, if there is no movement with respect to economic fundamentals -- that is, increased growth in Japan and West Germany -- the only other way to deal with it, perhaps, would be a recession in the United States, which, of course, is unacceptable to everybody."

A U.S. recession would tend to reduce the trade deficit because American consumers and businessmen would not be able to maintain their current high levels of imports. But it would also have serious negative implications for global prosperity.

Political issues dominated the meeting yesterday between Reagan and Japanese Prime Minister Yasuhiro Nakasone.

"Usually, when they talk, economic subjects dominate," said a Japanese official. "This time they didn't. They were subordinated."

High on Reagan's agenda, as he made clear in his public pronouncements, was the issue of forging a consensus on international terrorism in the wake of the April 15 U.S. raid on Libya and also of calling as much attention as possible to the Soviet nuclear reactor catastrophe.

Japanese Foreign Minister Shintaro Abe said in a recorded television program that will be broadcast here today that there are plans to make a special summit statement on international terrorism and the nuclear accident, which has spread higher than normal levels of radiation across wide areas of Europe.

A Japanese Foreign Ministry official said after the Reagan-Nakasone meeting that the Japanese "feel sympathy for the circumstances" that led to the U.S. raid on Libya. This was a slight strengthening of previous Japanese statements on this issue. Japan has taken no action against Libya, unlike some European allies who have ousted Libyan diplomats.

The terrorism issue also occupied 20 minutes of an hour-long meeting between Reagan and Italian Prime Minister Bettino Craxi. Italy persistently has refused U.S. requests to impose economic sanctions against Libya, a principal supplier of oil.

Both U.S. and Italian officials said that Reagan had asked Italy to reduce its dependence on Libyan oil. An Italian official said that Craxi urged Reagan "not to feel alone but not to act alone" against Libya, but that Craxi reiterated his opposition to economic sanctions.

A U.S. official said that Craxi also pointed out "the continued presence" of U.S. oil companies in Libya and said Reagan acknowledged that this was true.

In an effort to soft-pedal their differences and achieve a more united front on the terrorism question, U.S. officials have praised the recent action of European countries in expelling Libyan diplomats and muting their criticisms of the raid. But a senior U.S. official reaffirmed Reagan's determination to order further military operations if an act of terrorism can be traced to a particular government.

"When it comes to terrorism, no country is a fortress," Reagan said in his radio speech. "The death of innocent people at the hand of terrorists . . . is everybody's business, a threat to the liberty and well-being of all people."

Previously, Reagan has minimized the importance of a statement against terrorism at the economic summit, saying that private and frank discussions and the sharing of intelligence information were more important. But U.S. officials said they are now seeking some kind of joint statement, and Reagan said in his radio speech, "We must and will stand as one against the enemies of civilization."

The president's strongest statements yesterday, and those of other White House officials, were reserved for Soviet conduct in the wake of the reactor fire.

"The Soviets' handling of this incident manifests a disregard for the legitimate concerns of people everywhere," Reagan said. "A nuclear accident that results in contaminating a number of countries with radioactive material is not simply an internal matter."

A senior U.S. official said of the reactor fire in an interview that the United States "can use this to show why we're so insistent and will be insistent on verification" in arms control agreements.

During a day of briefings, U.S. officials repeatedly called attention to the Soviet nuclear accident while simultaneously denying that they were exaggerating the danger.

Responding to a question about whether the Reagan administration was "gloating" over the Soviet predicament, White House spokesman Larry Speakes said that the president's first word on the subject was an expression of sympathy in a personal letter to Gorbachev.

But Speakes devoted a major portion of his briefing to the hazards of radiation in other countries because of the Soviet accident. He said also that the incident may have helped Europeans form "a realistic understanding" of Soviet conduct.

The Japanese government announced here this morning that a special meeting had been convened headed by a Cabinet minister after radioactive contamination was detected in rain over areas of central Japan, according to a Foreign Ministrey spokesman. Japan said it was advising persons not to drink rainwater and to wash vegetables thoroughly. The radiation levels, believed to be a result of the Chernobyl accident, were not high enough to harm humans, according to officials quoted by the Kyodo news agency.

Secretary of State George P. Shultz, asked why the U.S. government pressed its criticism of the Soviets, responded that fallout in Europe menaced U.S. citizens "and it is the responsibility of the American government to look after the health and welfare of U.S. citizens."

But Shultz also used the incident to jab at Gorbachev.

Asked whether Gorbachev's response after the reactor fire indicated whether he was different from earlier Soviet leaders, Shultz said, "Well, he hasn't been forthcoming with information about this accident, and, so far as we can see, knowledge about it is far, far less than knowledge about it right here. So that doesn't look like an example of more openness."

U.S. officials said that Reagan will raise both the terrorism issue and the question of Soviet conduct on the nuclear incident in separate meetings that he will hold with British Prime Minister Margaret Thatcher and West German Chancellor Helmut Kohl shortly after they arrive here today, when the 12th economic summit formally opens.

In a speech to the U.S. Chamber of Commerce here yesterday, Reagan defended his domestic economic policies, particularly his insistence on tax reduction, and pledged to "work to open markets abroad" and "to resist protectionist pressures at home." But a senior U.S. official who briefed reporters after the Reagan-Nakasone meeting reported no progress on the thorny issue of the yen-dollar relationship.