MEXICO CITY-Two Cuban diplomats were detained for a week and then expelled from the country last month for violating Mexican security regulations, government sources have disclosed.

Mexican officials said the two diplomats were arrested as they handed explosives to two men the Mexicans described as anti-Castro activists from the United States. They did not explain why Cuban officials would be supplying enemies of Cuban President Fidel Castro.

The case has been kept out of the Mexican press. "To release information publicly would simply play into the hands of those who would like to see Mexican-Cuban relations suffer," one government official said.

This view reflects the notion, widespread in official circles, that the case was a "provocation" - that the Cuban diplomats may have been the victims of a trap designed by opponents of Castro to embarrass Cuba.

The incident represents a serious violation of an informal but usually scrupulously observed agreement between Cuba and Mexico not to engage in this sort of clandestine activity on each other's soil, according to Mexican officials. The agreement has helped to cement the friendship between the two countries, Mexican authorities said.

Envoy 'Crestfallen'

Fernando Lopez Muino, Cuba's longtime ambassador in Mexico, was described by one official as "crestfallen" and "deeply embarrassed" when he was presented with evidence in the case.

The ambassador insisted earlier that the police turn the diplomats over to him, and he had defended their activities in the apparent belief that they could not be engaged in any breach of Mexican security. He was reportedly not informed of their arrest until the fourth day of their detention.

Officially, neither Mexican nor Cuban officials will comment on the incident, though Mexican sources spoke privately and guardedly about it and a U.S. source provided confirmation.

The incident reportedly is being taken seriously by government officials and is said to have been brought to the attention of President Miguel de la Madrid, who is described as profoundly displeased.

At the moment of the exchange, the four men were arrested by agents of the Federal Security Directorate. The accounts indicate that Mexican security agents had foreknowledge of the planned exchange.

The arrests took place on Sept. 1, the day on which the Mexican president customarily rides in an open car from his residence to the National Palace in the center of the city. The city is converted into a virtual garrison, a Mexican official said, by the presence of thousands of extra uniformed and plainclothes agents guarding against the possibility of a disturbance.

"It's the worst possible day to try something, because the streets are so heavily guarded," the official said. "It's stupid, and the Cubans do not have a reputation for being stupid in these matters."

According to several accounts, the two Cuban diplomats left the apartment they shared early on the morning of Sept. 1 and drove aimlessly around the city. At some point, they got out and took a subway train, then changed trains to avoid being followed.

They ended up at a bus station on the north side of the city, where they attempted to hand over a quantity of explosives in a small leather case to two other men.

"It's not the amount of explosive that counts," a Mexican source familiar with the case said, "but the fact that it was highly refined explosive material."

One official said the arrests provoked a dispute between the Foreign Relations Ministry, which pointed out that the Cubans held diplomatic immunity, and the Ministry of Government, which is responsible for internal security and wanted the men held indefinitely.

One of the Cubans was described as a courier who had arrived in Mexico only the day before and thus was not accredited as a diplomat here. Both men carried Cuban diplomatic passports.

Mexican officials believe that Vie courier brought the explosive material into the country.

The two men, whose names could not be learned, were released on the night of Sept. 7 on the insistence of Mexican Foreign Minister Bernardo Sepulveda.

"They were taken straight from jail to the airport, not even given a chance to grab their personal belongings, as a way for us to show our displeasure over the entire matter," a Mexican official said. "This sort of thing is considered very serious."

The two were put aboard a Soviet-made Antonov airplane from Havana at a private hangar at Benito Juarez International Airport.

It was also not clear when and under what circumstances the two men described as anti-Castro activists were released, but they were sent back to the United States, according to Mexican authorities.

According to diplomatic sources, Cuba attaches great importance to its relations with Mexico. Cuba's embassy here, a modern building in the fashionable neighborhood of Polanco, is one of the largest Cuba maintains anywhere in the world.

U.S. diplomats consider the embassy one of the main outposts of Cuban espionage activity in Latin America and an important connection for Cuban involvement in Central America.

However, the Cubans maintain a low profile in connection with Mexico's internal affairs, hoping to court favor with the Mexican government.

An important example of the two nations' good relations is Central America, where Mexico has generally been in accord with the Cuban view that the conflict in the region arises from internal conditions rather than Cuban- or Soviet-inspired subversion.

The diplomatic affinity between the two countries extends to the economic field. On Sept. 5, when the two Cuban diplomats were in jail here, Mexican Treasury Secretary Jews Silva Herzog was in Havana to sign an agreement opening up two lines of credit totaling $55 million to allow Cuba to increase its imports of Mexican goods.