Costa Rican President Oscar Arias, principal author of the five-nation Central American peace initiative, asked Congress yesterday to "give peace a chance" and made clear his belief that the United States should halt aid to the Nicaraguan contras while negotiations for a peace accord are under way.

Arias spoke at the invitation of House Speaker Jim Wright (D-Tex.) to an unofficial but packed gathering of House and Senate members in the House chamber.

The meeting was unofficial because it occurred before Congress formally went into session. Wright agreed to this arrangement after the administration objected privately to his invitation to Arias, but in all other respects it was tantamount to a joint meeting of the two chambers.

Arias, who met with President Reagan before going to Capitol Hill, sought in his speech and other statements here to avoid public disagreement with the president.

But at a news conference after his speech, Arias acknowledged under persistent questioning that his views about the contras are directly counter to those of the administration.

"I think the military support to the contras has been the main excuse for the Sandinistas to do all they have done in the past -- to abolish individual freedoms, to abolish pluralism, to make of Nicaragua a more dictatorial society," Arias said.

"It's been the excuse for the failure of the Marxist experiment in the Nicaraguan economy. That is why I think we now should give the Sandinistas an opportunity . . . to comply with all the provisions of the Guatemala accord," he said.

His speech made no reference to the contras. Nevertheless, it was a barely disguised appeal for Congress to resist Reagan's calls for new contra aid until after the Nov. 7 deadline for reaching a peace agreement. In fact, he said, Central Americans should not be held to that deadline if the talks are showing signs of progress.

"Some steps may be taken before those deadlines expire; others may require a longer period," Arias said. "We will not fall into a trap set by someone who shows us a calendar every day, anxious to bury the last hope . . . . As long as there is a will to succeed, hope should never be lost."

His argument underscored the degree to which the peace plan, signed by the Central Americans in Guatemala Aug. 7, has widened the controversy over aid to the contras fighting the Marxist Sandinista government.

The administration, while publicly welcoming the plan, has expressed concern that it does not contain sufficient guarantees that Nicaragua will stop receiving military aid from communist countries and move toward democracy.

Reagan and Secretary of State George P. Shultz, arguing that continuing contra pressure is necessary to make the Sandinistas bargain in good faith, have said they will ask Congress for $270 million in new contra military aid after current funding expires next Wednesday.

The administration, in its responses, sought to play down differences with Arias over this central issue. After the Reagan-Arias meeting, a senior administration official, briefing on condition that he not be identified, stressed points of agreement between the two presidents so heavily that some reporters suggested that he was giving a one-sided and distorted picture.

The official, who gave his briefing before Arias' appearances on Capitol Hill, argued that the important results of the White House session were agreement that the contras should have a role in cease-fire negotiations and Arias' support for an interim appropriation of $3.5 million to continue humanitarian aid to the contras for 30 days after the fiscal year begins Oct. 1.

Later, after Arias' statements had underscored the depth of his disagreement with Reagan on contra aid, other officials denied that the senior official had tried in the briefing to be misleading about the point of Arias' visit.

"It's not news that Arias opposes aid to the contras," one official said. "We differ on the role that military aid should play in this process, but we're in fundamental agreement on the goals we want in Central America."

He added that the senior official had correctly identified "real news" from the White House meeting.

He argued that Arias, by agreeing that cease-fires must be mutually negotiated and involve the contras, had helped to expose the Sandinistas' announcement in Managua yesterday of a unilateral, limited cease-fire as a propaganda ploy.

And, he added, by supporting the $3.5 million in humanitarian aid, Arias may be making it less urgent for the administration to confront Congress with its $270 million request.

Congressional reaction to the speech and to an hour-long private meeting with Arias afterward was generally positive. Wright, whose collaboration with Reagan this summer on a bipartisan peace initiative helped to spark the Arias plan, called it "a hopeful report on the peace process."

While the greatest enthusiasm was expressed by Democrats, who control the House and Senate, Rep. William S. Broomfield (R-Mich.), senior minority member of the House Foreign Affairs Committee, said, "I think both sides were pleased with his responses, the liberals and conservatives alike."

The sharpest dissent came from Rep. Jack Kemp (N.Y.), a Republican presidential candidate and strong proponent of contra aid. He warned that "it will be very difficult to get assistance to governments in Central America -- particularly Costa Rica -- if there is a lobbying effort by President Arias against U.S. assistance to the freedom fighters in Nicaragua."

House sources have said that the White House, after initially endorsing Wright's invitation to Arias, reversed direction and sought to have him cancel the event or move it to a location outside the Capitol.

The White House also waited until nearly the last minute to invite Arias, who is in this country on a private visit, to meet with Reagan.