Cuban President Fidel Castro has charged in an unusual letter to The Washington Post that reports of the presence of Cuban troops in Nicaragua are part of a "campaign of falsehood and lies" by the U.S. government "aimed at setting the stage" for aggressive U.S. actions against Cuba.

In a two-page letter to the editor received by The Post yesterday, Castro denounced a report last Oct. 19 by syndicated columnists Rowland Evans and Robert Novak on Cuban troops in Nicaragua as "truculent and absolutely false." Castro also charged that the United States had informed "third countries" that it had proof of Cuban troop activities in Nicaragua without responding to Cuban demands that it release evidence to support the charge.

The Evans and Novak column, published in The Post and other newspapers, reported that some 500 to 600 elite Cuban troops were flown secretly to Nicaragua in mid-September. The account said the troops "may be aimed at setting up a revolutionary Marxist government in eastern El Salvador," and suggested that the Cubans may have been behind the destruction last month of a strategic bridge over the Lempa River in eastern El Salvador.

While the Cuban and Nicaraguan governments denounced the report, the State Department repeatedly has avoided denying that it has evidence of the Cuban troop movement, raising the ire of both governments and apparently leading to Castro's letter to The Post. Yesterday, a State Department spokesman refused to comment on the letter, while Novak released a statement saying that the columnists "stand behind all information contained in the column."

President Reagan said twice in his news conference yesterday that the United States has no plans to intervene militarily in Central America.

The Cuban leader's letter reflected what appears to be a conviction in Cuba, fueled by public statements by Secretary of State Alexander M. Haig Jr., that the Reagan administration is considering military action against Cuba or Nicaragua because of those countries' alleged support of leftist guerrillas in El Salvador. It also marked one of the occasional efforts by Castro to present his case to the American media at a time when his relations with a U.S. administration are tense.