A year to the day after Nicaragua's bloody revolution ended, this city's streets erupted again -- in celebration.

More than a quarter of a million people gathered in the vast new July 19 Plaza to hear their Sandinista leaders affirm their strength and independence. The Sandinistas, however, were somewhat upstaged by visiting Cuban President Fidel Castro, who applauded U.S. aid to Nicaragua, said the United States could be spending more money here and called Ronald Reagan a "threat to world peace."

Speeches praising the Sandinista victory over the dictatorship of Anastasio Somoza were also given by Grenadan Prime Minister Maurice Bishop and former Venezuelan president Carlos Andres Perez, whose government supported the Sandinista forces.

On the first anniversary of their triumph, the Sandinistas set out to demonstrate what they have accomplished and present their revolution as an example to Latin America and all the developing world.

But by failing to announce a date for municipal elections, as the still-influential private sector here has demanded, the Sandinistas may have laid the groundwork for a major internal crisis in the coming weeks.

Castro, making his first trip outside Cuba since the spring refugee crisis, attempted not to upstage the Sandinistas, but cries of "Fidel! Fidel!" erupted for long periods throughout his speech.

The Cuban leader, who is looked on almost as the godfather of the Nicaraguan revolution, told reporters at a party last night that he believes the United States "is following in its foreign policy a line that is more intelligent, more constructive" than in the days when his own revolution was young.

Today, although he alluded to the need for revolutions such as Nicaragua's throughout this area, where many repressive governments still are ostensibly supported by the United States, Castro was careful to avoid direct attacks on the Carter administration. The U.S. delegation here is headed by Ambassador to the United Nations Donald McHenry.

Castro's strongest criticism of the current U.S. role in Nicaragua was merely a suggestion that the $120 million in aid provided so far is not nearly enough in light of the donor's wealth.

The shadow of Ronald Reagan and his potential victory in the U.S. presidential election loomed at today's rally. Castro vehemently attacked the Republican Party platform as "a threat to peace" that could cause people "to practically fight to save peace."

Daniel Ortega, speaking for the Nicaraguan ruling junta, described Reagan as "a brother of Somoza." Nicaragua would not be frightened by whatever "garbage" Reagan might throw, Ortega said, looking out at the multitude beneath raised black-and-red Sandinista flags and rank upon rank of soldiers and militia.

Perez argued strongly in his address for the maintenance of pluralism and democracy here but such concepts often were lost amid rhetoric from other speakers emphasizing the importance of revolution.

Bishop of Grenada set the tone when he called for revolution like Cuba's, like his own and like Nicaragua's in nearby El Salvador, Guatemala and Honduras.

U.S. Ambassador Lawrence Pezzullo has maintained that whatever the tone of today's celebration, the realities faced by Nicaragua and its leaders are unchanged, requiring the same sort of moderation, pragmatism and compromise that he said the Sandinistas have demonstrated in growing measure during the last year.

"It's like the football team that won the Saturday game," Pezzullo said. "They're going to have their dance." The Sandinistas "have to sit down and run the country on Monday morning."

Businessmen in Nicaragua, on whose fate the United States has pegged much of its support, see the celebration as a more significant determinant of the direction the revolution is going to take.

Since the resignation of two non-Sandinistas from the governing junta in April, the private sector and the Sandinistas have come a long way toward reaching an understanding about their mutual contributions to Nicaragua's development.

Most confiscations of land have stopped. The state of emergency giving the government extraordinary powers has ended. A law gives private citizens the right to take the government to court and the independent newspaper La Prensa has reopened as the private sector demanded.

The appointment of two new members to the five-person junta, both of them respected independents, was seen as a major step toward moderation. The subsequent approval by the U.S. Congress of major financial aid marked the warming of relations with Washington.

At the same time, businessmen have played an increasing role in the new legislative-advisory Council of State and, most important for this devastated economy, they have begun to put their farms and factories back into full-scale production.

But the failure to announce a date when Nicaraguans could expect to go to the polling places, despite Ortega's statement that the Sandinistas' commitment to pluralistic democracy and elections remains strong, is not likely to satisfy the private sector here.

Many influential businessmen, prior to today's celebration, suggested that members of the private sector would resign from the Council of State if an election date were not set by today.

A preoccupation with security has been building here for the past few weeks.All cameras, tape recorders and radios were searched at the Managua airport.

One cab driver said he and his colleagues were required to report where they drove correspondents, and three journalists in a private car who visited the house of an opposition leader were tailed from the time they left the leader's home.

Sandinista junta member Moises Hassan, asked to name the major accomplishments of the first year, listed the organization of the state security forces, the Army, the militia and the police.