MADRID, OCT. 10 -- The Guatemalan government and leftist rebels agreed during three days of secret meetings to remain in contact but did not set a date for a new round of peace talks, spokesmen said today.

In separate news conferences, government officials and guerrilla leaders said their meetings in the Spanish capital were constructive despite their failure to reach an accord to end Central America's oldest insurgency.

They said contacts would continue through an unnamed intermediary.

Guatemala's three-man delegation said the rebel demands would be passed on to the government of President Vinicio Cerezo, explaining that they had no mandate to negotiate in the talks, which ended yesterday.

Guerrilla leader Rodrigo Asturias, known as "Commander Gaspar Ilom," called the meeting a "first step" toward reaching peace.

He said rebel demands included an investigation into human rights abuses and the disappearance of 38,000 people.

He denounced the influence of the Army in civilian affairs despite the country's return to democracy in January 1986 with elections that brought Cerezo to power.

Roberto Valle, first vice president of the Guatemalan Congress, said, "We know there are murders {in Guatemala} but is there any country where that is not the case?"

The government of Cerezo and the left-wing Guatemalan National Revolutionary Union agreed last week to discuss ways to end the 26-year insurgency, which is believed to have taken 100,000 lives.

The talks were part of an agreement signed Aug. 7 by the five Central American presidents. The accord set a Nov. 7 deadline for cease-fires aimed at national reconciliation in Nicaragua, El Salvador and Guatemala.