LA CRUZ, COSTA RICA, OCT. 20 -- Harried Costa Rican authorities here are struggling to feed and shelter 636 Nicaraguans who surged across the border Sunday in the largest single-day influx of Nicaraguan refugees to this country.

The exodus came as President Oscar Arias, who recently won the Nobel Peace Prize, is busily promoting a regional peace plan that he wrote in part to lessen the flow of Nicaraguan exiles into Costa Rica.

Since early September the two governments have allowed family members living on opposite sides of the border to mingle on Sundays at the Penas Blancas crossing point. Nicaragua's leftist Sandinista government sponsored the reunions hoping relatives would meet with rebel fighters, called counterrevolutionaries or contras, and urge them to return to civilian life in Nicaragua.

But many Nicaraguans here said they heard Nicaraguan President Daniel Ortega speak of the border openings for the first time in a Sandinista Radio broadcast last week. The announcement came at a time when Sandinista recruiters were stepping up a new conscription campaign in southern Nicaragua, they said. Thousands of Nicaraguan young men have left the country to avoid being drafted.

A first group of 100 or so Nicaraguans pushed past a handful of Sandinista border guards early in the morning and clambered over a chain-link border fence, witnesses said. The Sandinista guards tried to stop them by shooting in the air, but no one was injured.

At midmorning Radio Impacto, a procontra Costa Rican station, broadcast to southern Nicaragua that some Nicaraguans had already made the crossing, according to refugee William Mercado, who heard the newscast in his home. Mercado said he sped out the door and hopped a bus to the border with nothing more than the clothes on his back.

Refugees said guards on both sides finally left the border open most of the morning so they could cross freely. Several dozen Nicaraguans paid truck drivers to drive them across, refugees said.

The Nicaraguans said they were tired of severe food shortages, starvation wages and a recent military draft call.

"There's no rice, there's no soap. If I feed my children twice a day, I can only eat once myself," said Marta Alicia Grillo, an emaciated woman carrying a 2-year-old son. Grillo's husband, who also sought refuge, was called up to the Army in early September and had been in hiding for a month in their home town of Rivas, she said.

Another recent draftee, Jorge Antonio Quiroz, said he heard of Arias' peace plan but did not believe it would succeed soon enough to help him avoid military service.

"As long as that Sandinista government stays in power, it's a lie to say there will be peace," Quiroz said.

Student Melania Cordero said she left her sobbing mother at the border gate in Nicaragua to look for work as a maid in Costa Rica.

"In this country you can buy what you want and the stores have what you want to buy," Cordero explained.

In La Cruz, 15 miles south of the Penas Blancas border station, frantic Costa Rican Red Cross volunteers prepared a tent campsite for the 452 men among the refugees, being held temporarily in the cramped Sea Gull discotheque. Health workers hurried to interview and vaccinate every Nicaraguan, fearing the spread of disease.

Costa Rica now hosts about 25,000 Nicaraguans registered with the United Nations as refugees. But tens of thousands of others are living here on their own. Arias has sounded an alarm, saying a total of 100,000 Nicaraguans live in Costa Rica and citing them as a threat to Costa Rica's national integrity.

Three weeks ago Costa Rica and Nicaragua signed an agreement with the United Nations to repatriate Nicaraguan refugees who wish to go home. The first 33 Nicaraguans returned to their country Oct. 14 under the plan, a U.N. spokesman said.