UNITED NATIONS, N. Y., Dec. 11 - There was more mystification than excitement here today after a shell exploded in the East River just outside the United Nations.

But there was one brief moment of intense confusion-when the blast reverberated throughout the United Nations complex and a fountain of water could be seen 200 yards offshore.

At that moment, Ernesto Che Guevara, a key associate of Premier Fidel Castro of Cuba, was at the rostrum of the General Assembly bitterly attacking the United States. The explosion was clearly heard in the Assembly Hall, but Major Guevara ignored it.

In the Security Council, Foreign Minister Paul-Henri Spaak of Belgium was bitterly accusing African delegates of racism. He never paused in his speech, but the blast was heard so loudly in the council chambers that spectators jumped in their seats.

Later, Major Guevara treated the incident as a joke. As he strolled into the delegate lounge for coffee, he remarked that the explosion "has given the whole thing more flavor." He was asked why he thought the shell might have been fired.

Wearing his usual green-fatigue uniform and waving a Cuban cigar, he replied:

"Why not ask the man who planted the bomb? I'm not interested."

The immediate thought of those who heard the blast was that some nearby building had been dynamited for demolition.

But those who saw a geyser rise about 15 feet in the East River knew there was more to it than that. Rumors began to spread that an underwater bomb had been set off to distract the police, who were watching anti-Castro pickets in front of the Secretariat Building.

There was speculation that with the detonation of the explosive, the Cuban demonstrators planned to rush the gates of the United Nations enclave, in an effort to enter the Assembly Hall while Major Guevara was speaking. But only two policemen were sent from First Avenue to the river to see what had happened.

Some early lunchers in the staff cafeteria were at tables overlooking the river. Some of the insisted that they had seen a small black object shot from across the shore in Queens.

It was known from previous experience that the presence of a prominent pro-Castro Cuban would bring the usual anti-Castro demonstrators. Thus the police were out in full force well before the pickets arrived.

A split second before the explosion, a woman carrying a knife broke away from the demonstrators, raced across First Avenue and succeeded in vaulting a picket fence onto the grounds of the United Nations. Pursued by three policemen, she ran for the row of flags.

After a struggle in which she scratched the face of a policeman she was subdued and taken to the East 51^st Street station.

The woman identified herself as Molly Gonzales, 24 years old, of 25 Union Square, Jersey City. She told police she came to the United States two years ago. Miss Gonzales was booked and charged with felonious assault and with the possession of a knife.

Employees and members of the Secretariat whose windows look out on First Avenue and who had heard the sound of the explosion connected it with the sudden action on the street below as the police chased the woman.

Their belief that a bomb had been set off near the front of the building was reinforced when security guards and police sealed all entrances and gates and for a long time allowed no one to enter or leave the grounds.

For some hours after the explosion, there was a buzz of conflicting rumors in the corridors and meeting places of the buildings. Somehow, the news of an anonymous phone call to the police warning of a bomb in the building had become known and was spread.

Shortly after the blast went off there was speculation that the police had indeed found a bomb somewhere in or near the United Nations buildings and that, rather than risk disarming it, they had thrown it in the river, where it went off harmlessly.

This theory was clearly favored by the majority until the news of the finding of the rocket launcher across the river reached United Nations Headquarters.

Just as the atmosphere was returning to normal within the building, there was a renewed flurry of excitement outside.

The Cuban demonstrators, still kept by the police to the sidewalk opposite the United Nations, began singing the Cuban national anthem as they marched up and down. One demonstrator, meanwhile, managed to evade police notice. In the gathering dusk, he made his way to the base of the pole flying the Soviet flag.

He succeeded in cutting the flag down but before he could throw it to another Cuban who was waiting to burn it, he was grabbed by the police. Undaunted, the demonstrators pulled out a "homemade" Soviet flag to which they then set fire.