The ruling Sandinistas and the U.S.-backed Contras opened a second day of direct negotiations Tuesday with both sides momentarily putting aside the bitterness of seven years of civil war.

The unprecedented three days of negotiations began Monday in Sapoa, a border post 85 miles southeast of Managua on the Costa Rican frontier.Both sides said they would honor a cease-fire for the duration of the talks, the first ever on Nicaraguan soil and the first in which the two sides have met without a mediator.

Neither delegation commented publicly Tuesday as they entered the customs house in Sapoa, where they were meeting on opposite sides of an oblong table. The talks began Monday on surprisingly courteous terms, despite more than a week of heavy fighting along the Nicaragua-Honduras border, where the Sandinistas were accused by Washington and Tegucigalpa of intruding into Honduran territory to pursue Contra units and to destroy rebel base camps.

President Reagan ordered 3,200 U.S. troops to Honduras in response to the alleged Nicaraguan incursion, but by Monday Washington and Honduran officials in Tegucigalpa said the Sandinistas had withdrawn. Managua denied ever having invaded Honduran territory.

Nicaraguan Defense Minister Humberto Ortega, leader of the government delegation, would not disclose the government's proposals at the first day of closed-door talks Monday but emerged from the six-hour session saying, "This is a good beginning. We are not inflexible, and after studying their plans we will be able to provide more details."

"I believe this has been an important round of talks, and we hope to contribute to the conditions of achieving a cease-fire and stopping the spilling of blood which all Nicaraguans deserve," Ortega, brother of Nicaraguan President Daniel Ortega, told reporters after the meeting.

Contra leader Adolfo Calero said the discussions were "frank, clear, direct, and courteous in tone."

"We have agreed to continue tomorrow to discuss our proposal and that of the Sandinistas. We feel we accomplished something today," Ca-lero said.

Alfredo Cesar, another leader of the rebel movement, known as the Nicaraguan Resistance, said the Con-tras proposed a 30-day truce "in exchange for total amnesty and total freedom of expression."