Directors of Nicaragua's opposition newspaper La Prensa today called its closure by the Sandinista government a "black moment" and said an appeal would be made to the country's highest court.

"We always maintained that the day that La Prensa was suppressed, the last breath of freedom in Nicaragua would be stopped," said Carlos Holmann, executive vice president of La Prensa, at a press conference.

Holmann and other directors of the paper indicated they did not think an appeal to the Supreme Court of Justice would have much effect. They said previous appeals over censorship had met with no response.

In response to the approval Wednesday by the U.S. House of Representatives of $100 million in military and nonlethal aid to Nicaraguan insurgents, who are known as contras, Nicaraguan President Daniel Ortega said yesterday that security measures would be intensified, a state of emergency would be more strictly applied and all mechanisms would be used to control any internal front for the contras.

The address followed a government order yesterday to close the newspaper for an "indefinite period of time." It was delivered in a three-line letter signed by government censor Capt. Nelba Blandon.

"It could be the opening salvo," a diplomat said. "No one thinks it will stop there. One has to anticipate a direct attack on what they consider treason," he said.

So far, no other action has been taken against two opposition political parties and a private business organization, according to members interviewed. A worker at offices of the Roman Catholic Church hierarchy reported no activity.

"We are waiting," said Virgilio Godoy, president of the opposition Independent Liberal Party. "They closed La Prensa, so why won't they close everything else? Their first step was a strong one."

At today's press conference, members of the Chamorro family, who run the 60-year-old newspaper, and its employes rejected charges that they function as agents of the Reagan administration. They passed out copies of an editorial that criticized the House approval of $100 million in aid as a measure that would "increase . . . the levels of anguish, hunger and sacrifice in the lives of Nicaraguans."