WASHINGTON, April 1 -- Seventeen heavily armed Cuban exiles planning to attack a Soviet tanker off Cuba were seized yesterday by a. British force on a solitary Caribbean islet.

The men and a 35-foot twin-engined motor launch carrying a cannon and two heavy machine guns with explosive bullets were taken in custody at the tiny island, called Norman Key, a sliver of land 200 miles north of Cuba.

The seizure apparently occurred when the raiders came ashore to prepare for the run.

However, a second rebel boat belonging to the "Cuban anti-Communist Army," a small anti-Castro group based in Miami, was reported to have eluded capture and to be sailing toward Cuba with a cargo of weapons for underground fighters on the island's northern coast.

The capture of the 17 was announced this afternoon by refugee spokesmen in Miami. It was confirmed by the British and United States Governments.

It was understood that word of the seizure of the party, led by Maj. Evelio Duque, known as a leader of anti-Castro forces in the Cuban mountains, had been radioed to Miami by the skipper of the boat carrying arms for rebels.

The capture was made by a detachment of Bahamian policemen who flew to the island. The captives were placed aboard the British frigate Londonderry to be taken to Nassau, the capital of the Bahamas, where any action to be taken would rest with the Governor of the Bahamas.

The capture was apparently the first result of an agreement, worked out late last week by Washington and London, to cooperate in preventing raids by opponents of Premier Fidel Castro.

The agreement primarily calls for an exchange of intelligence information on the rebels. It was understood tonight that the United States had advised the British that a suspicious situation had developed around Norman Key.

But, despite rebel charges, an Administration spokesman said, United States forces did not participate directly in the seizure.

Two raids last month led to serious concern here that a crisis could develop between the Soviet Union and the United States.

The Soviet freighter Lvov was attacked March 17 by commandoes of the Alpha 66 group. On March 26, a group known as "Commando L" attacked the freighter Baku.

The Soviet Union charged the United States with responsibility for these raids and hinted that it might have to arm or escort merchantmen to prevent new attacks.

Over the weekend, the Administration took measures to halt the rebel raids on the ground that they violated laws of the United States and interfered with its foreign policy. Among other steps, 25 anti-Castro leaders were ordered confined to the area of Dade County, which surrounds Miami.

Strengthened air and sea patrols of the Caribbean and the plan to cooperate with Britain completed the arrangements.

While Cuban exiles reacted with a mixture of anger, defiance and gloom, leaders in the Senate said they approved.

The Majority Leaders, Senator Mike Mansfield of Montana, said that much as he sympathized with the anti-Castro groups, "they cannot take the law into their own hands."

The Republican leader, Senator Everett McKinley Dirksen of Illinois, described rebel raids as having "no effect except to stir up a lot of trouble."

Senator Wayne Morse, Oregon Democrat, who is chairman of the Senate subcommittee on Latin America, said that if the United States found itself again confronting the Soviet Union, "it should be on behalf or ourselves and not because we were forced into it by a hot-headed group of guests in our own house."

The agreement with Britain to seek to control the rapidly increasing numbers of exile actions was deemed necessary because many raiders use British islets and keys as staging areas.

Britain's role hinges on eliminating "illicit" use of British territory.

But the immense complexity of halting the raids is indicated by the fact that there are hundreds of keys and rocks under the British flag in the Caribbean. In fact, a British Embassy spokesman said, "they were never counted."

By using these mostly uninhabited islets and rocks as bases the exiles avoid charges that they mount raids from United States territory and thereby violate neutrality laws.

In practice, motor launches and military equipment are generally stored on one of the keys after they have been brought from the United States mainland. The raiders then arrive in another boat from Florida, often in a fishing boat, and transfer to their vessel on the key.

The details of yesterday's action at Norman Key were skimpy and contradictory.

Dr. Orlando Bosch, a spokesman for the "Cuban anti-Communist Army," contended that the capture of Major Duque's boat took place in international waters. He called it an "act of piracy."

But a communique issued by the Bahamas Government in Nassau reported the detention of 17 men who landed yesterday on a key in the Exuma Islands.

Both the State and the Defense Departments seemed at first completely unaware of the incident. A State Department spokesman later confirmed it.

In the evening, Lincoln White, a State Department spokesman, said that the Bahamian police force had gone to Norman Key to investigate a report provided by the United States.

During the follow-up, Mr. White said, one rebel boat landed on the key and Major Duque and his men were taken into custody.

When the Londonderry arrived, Mr. White said, the raiders were transferred to the frigate.

The case is now a "British matter," he said.

Despite the capture of the rebel boat, the outlook indicated Cuban exile groups would seek to continue their raids.

In Miami, Dr. Bosch said that the second boat of the "Cuban anti-Communist Army" was sailing to Cuba and "if it is taken it will have to be taken by force."

LONDON, April 1--The Admiralty said marines from the Londonderry had gone ashore with the Bahamian policemen to capture the Cubans, who were armed but put up no resistance. No shots were fired.

The Cubans' launch, now manned by men from Londonderry, is being taken to Nassau.

The Londonderry sailed from Bermuda on Saturday bound for the tiny island, which is part of the Exuma group.

There are hundreds of British islands in the Caribbean.

Cuba protested to Britain on Feb. 23, charging the islands were being used by anti-Castro refugees. The protest has no yet been answered.

A Foreign Office spokesman said today that Britain was determined to do all she could to prevent the illicit use of British territory in the Caribbean.

The spokesman was asked whether this meant Britain would seek to keep commandos of the Castro regime from kidnapping Cuban exiles as well as keeping the exiles from attacking Cuba.

"We would be concerned about the use of British territory for any illicit purpose," the spokesman replied.