The main anti-Sandinista rebel force today declared a 12-hour cease-fire for Sunday's election to allow Nicaraguans to vote without danger.

The truce announcement, made here by rebel spokesman Frank Arana, emphasized concern for Nicaraguan civilians who might be caught up in violence if guerrilla forces sought to disrupt the voting. This provided a public relations counterweight to recent discussions in Washington over a rebel manual compiled by the CIA listing "neutralization" of civilian Sandinista officials as a desirable tactic in some situations.

"We know that out of terror, survival and the daily necessity of feeding their families, many Nicaraguans will be obliged to cast their ballots in the electoral farce," said Arana, spokesman for the Nicaraguan Democratic Force. "Since our highest concern is to defend the civilian population, we are decreeing a cease-fire in place from 5 a.m. to 5 p.m. next Sunday."

This will be the first cease-fire since the rebels began attacks against the Sandinista government in the spring of 1982, he added.

Arana underlined, however, that the estimated 10,000 Nicaraguan Democratic Force guerrillas in Nicaragua "will be alert to repel any attack or advance that the Sandinista Army might want to make."

He made no reference to guerrillas along the southern border with Costa Rica under the command of Eden Pastora. But since the breakup this spring of his Costa Rica-based rebel group, the Revolutionary Democratic Alliance, Pastora's small force has been mostly inactive.

The Sandinista leadership repeatedly has charged that Honduras-based rebels, who were organized with CIA funding and advice, are seeking to disrupt next Sunday's vote by killing election officials and robbing civilians of their voter registration cards. A Sandinista candidate for the constituent assembly and an election official were released earlier this week after being seized by rebels in northeastern Nicaragua.

The election, the first since Sandinistas seized power in 1979, will choose a president, a vice president and a 90-member constituent assembly. Sandinista leaders had hoped that it would demonstrate a commitment to democracy and pluralism. But the country's main opposition parties have refused to participate, charging lack of political freedoms, and the ruling Sandinista National Liberation Front is considered a certain victor with no significant opponent.

Arana asserted that, contrary to the Sandinista objectives, the vote demonstrates that the Nicaraguan government is undemocratic.

Washington Post correspondent Robert J. McCartney added from Managua:

About 250,000 people crowded into a downtown plaza in Managua this evening to demonstrate support for the ruling Sandinista Front. Head of state and presidential candidate Daniel Ortega warned that an alleged U.S. spy flight yesterday over Nicaragua "presaged" U.S. military action "of greater scope" after the U.S. and Nicaraguan elections.

The crowd and the nine commanders of the Sandinista directorate waved black-and-red plastic flags of the front. Several government ministers and other officials performed chorus-line kicks on the grandstand to the tune of the Sandinistas' campaign song.