OSLO, OCT. 13 -- Costa Rican President Oscar Arias, the author of a peace plan for Central America signed in August by the region's five presidents, today won the 1987 Nobel Peace Prize.

Arias, the fourth Latin American to win the prize in its 86-year history, was cited by the Norwegian Nobel Committee "for his work for peace in Central America."

In San Jose, Arias told a group of reporters that "all five presidents of Central America deserve the award." He said he would use the estimated $340,000 in prize money to form a "foundation for Costa Rica's neediest citizens."

The announcement came as a surprise to many observers and is believed to have been decided only in the past few days by the five-member committee, appointed for six-year terms by the Norwegian parliament. Arias, 46, was not even mentioned on a rumored short list that included Philippine President Corazon Aquino, who led her country's return to democracy, and two perennial candidates, former U.N. peace forces organizer Brian Urquhart and the World Health Organization.

In its statement, the committee called the Arias plan, which is scheduled to take effect Nov. 7, "an outstanding contribution to the possible return of stability and peace to a region long torn by strife and civil war." It called him a "strong spokesman" for "democratic ideals, with freedom add equality for all." The monetary award and a gold medal are to be presented here by the king of Norway on Dec. 10.

President Reagan, asked to comment on the award as he left Washington today for a trip to New Jersey, said, "I congratulate him."

A statement issued later by the White House said that Arias "fully deserves the peace prize for having started the Central American region on the road to peace. This award should inspire all of us to renew our efforts to ensure that enduring peace and democracy eventually come to the region."

House Speaker James C. Wright Jr., who cosponsored a similar plan with Reagan before the Arias plan was approved, said the awarding of the prize to Arias would "impose a strong moral call on the U.S. to act to see that it {the peace plan} is implemented."

House Majority Whip Tony Coelho (D-Calif.) said the prize could end any hopes of congressional approval of an administration request for new aid to Nicaraguan rebels.

"This kills it; it's dead," Coelho said.

The award was questioned, however, by congressional supporters of administration policy in Central America, including aid to the rebels, who suggested that it was premature, and might unduly influence events in the region.

Rep. Jack Kemp (R-N.Y.) said, "They ought to save the peace prize until they see what happens in the future." House Minority Leader Bob Michel (R-Ill) said it was not clear whether the Arias plan was "the means to achieve" the administration's goal of "peace and freedom for the people of Nicaragua."

"I don't know that the Norwegians have got all that much to say about what goes on in Central America," Michel said.

The administration has offered only lukewarm backing for the peace plan, and is concerned that its popularity has undercut already dim prospects of winning congressional support for the $270 million requested for the Nicaraguan rebels.

In a speech last week to the Organization of American States, Reagan called the plan "a step in the right direction," but insisted that it "does not address U.S. security concerns in the region." The administration has said the accord is flawed because it does not commit the Sandinista government in Managua to the immediate withdrawal of Cuban and Soviet military personnel and immediate new elections.

Virtually every government in Latin America and in Western Europe has supported the accord, however. Although it is considered to have drawbacks, the Arias plan is seen as the first optimistic sign that Central America's various wars can be resolved at a regional diplomatic level, without direct outside military intervention.

The plan calls for an internal dialogue between each of the warring Central American governments and its unarmed political opposition, a cease-fire and amnesty for rebels and political prisoners, full freedom of speech and other peaceful political expression, and free elections held in accord with each country's constitution. It commits each of the five signatories -- Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica -- to request all outside governments supplying aid to armed rebels in the region to stop such aid.

The accord already has led to direct talks, so far inconclusive, between the governments of Guatemala and El Salvador and their armed opponents, and has brought the first tentative liberalization of the revolutionary government in Managua.

Nicaraguan President Daniel Ortega today expressed his "deep satisfaction" at the choice of Arias.

Nobel committee chairman Egil Aarvik denied that the prize was intended to be a political statement against U.S. policy.

"We fervently hope that the influence of what we've done will be positive, not only in Costa Rica, but in all of Central America and other countries," he said.

Referring to the more conciliatory aspects of Reagan's OAS speech, Aarvik said that "President Reagan himself, with some reservations, has approved of the peace plan."

Aarvik said the award was unusual because Arias had received only one nomination, by Swedish legislator Bjorn Molin. The list of contenders this year totaled 93 individuals and organizations, many of them supported by dozens of nominations and letters of support.

The nomination deadline for this year's award was Feb. 1. Arias had been mentioned as a contender for next year's award, because his most widely hailed achievement, the peace plan, came after the February cutoff.

But Aarvik said that Molin "obviously knew a lot about what {Arias} had done last year," and submitted a detailed report in support of the nomination. Since his election as Costa Rican president in 1986, Arias has been closely involved in the work of the Contadora initiative, the longstanding but unsuccessful effort by Mexico, Venezuela, Costa Rica and Panama to find peaceful solutions to the Central American conflicts.

One of his first acts as president was to shut down a U.S.-built clandestine airstrip in northern Costa Rica, used for supplying the Nicaraguan rebels, known as contras.

But while opposing the contra program, Arias has been no supporter of the Sandinistas. During a May 1986 Central American summit meeting in Guatemala, Arias later related, he confronted Nicaraguan President Daniel Ortega and told him, "The difference between you and the rest of us is that we are prepared to become leaders of the opposition, while you are not. You are not prepared to risk political power."

Still, the first drafts of what later became the Arias peace plan did not emerge until February, when he offered a 10-point proposal along the lines of an earlier Contadora plan, including amnesty, cease-fire and dialogue between the governments of Nicaragua and El Salvador and their unarmed opponents.

Asked today whether subsequent events, long after the Nobel nominating deadline had passed, had influenced the committee's decision, Aarvik said, "We are not operating the committee in a vacuum. We have windows to the world. We read newspapers and watch television."

The breakthrough in August, when five Central American presidents signed a peace plan and timetable for the first time, had "of course" influenced the committee decision, Aarvik said.

Aarvik said he expected the award to be "no more controversial than usual." He and others noted that earlier peace prizes had been awarded to politicians for their work on controversial initiatives still unproven at the time of the Nobel decision. They included former West German chancellor Willy Brandt, who received the 1971 prize for his efforts to bring East and West Europe together, and the 1978 award to Egyptian president Anwar Sadat and Israeli prime minister Menachem Begin for what became their 1979 peace agreement.

Arias is the fourth Latin American to receive the peace prize, which was one of five awards set up under the 1895 will of Sweden's Alfred Nobel, the inventor of dynamite.

In 1936, Carlos Saavedra Lamas, an Argentine, won the peace prize for mediating a conflict between Paraguay and Bolivia. The 1980 prize went to Argentine human rights campaigner Adolfo Perez Esquivel. In 1982, the prize was awarded jointly to nuclear disarmament campaigner Alva Myrdal of Sweden, and Mexico's former foreign minister Alfonso Garcia Robles.

Arias once studied medicine at Boston University and later was trained in law and economics at the University of San Jose, the University of Essex and London School of Economics in Britain. He is a protege of former president Jose Figueres, considered the father of modern Costa Rican democracy. Figueres led a revolution in 1948, after which he abolished the armed forces in favor of a police force.

In 1972, president Figueres appointed Arias planning minister in his National Liberation Party government. In 1979, Arias became secretary general of the party, a position he resigned in 1984 to seek the 1986 presidential nomination.

Washington Post staff writer Tom Kenworthy contributed to this report.