Spain's decision to attend the Havana nonaligned nations conference was defended here by House Foreign Affairs Committee chairman Clement Zablocki (D-Wis.), who said it was the U.S. interest to have allies present at the Third World forum set to start next week.

Despite the Spanish government's commitment to seek membership in NATO, the Foreign Ministry took the surprise decision to accept an invitation to attend the Havana conference as a guest. A seven-man team headed by the secretary of state for foreign affairs, Carlos Robles Piguer, will be the first Spanish delegation present at a nonaligned meeting.

Zablocki, who held talks with Robles Piguer yesterday, told reporters, "I have not been advised that our country has any objections" to Spain's attendance. "Personally I think it is a good decision."

Press reports here had said that Spain's decision had distressed Secretary of State Cyrus Vance, who met Spanish leaders in Quito, Ecuador, shortly before acceptance of the Havana invitation was made public earlier this month. [A State Department spokesman said, "We do not see Spain's attendance as guests as incompatible with Spain's strong Western orientation nor, we understand, does the Spanish government.]

Zablocki said he believed Spain could "give a productive input to Third World problems . . . It is very helpful to have an ally at such a conference."

The congressman is on a stopover in Spain at the head of an eight-member House delegation to attend a U.N. sponsored science and technology conference for developing countries in Vienna later this week.

Speaking to reporters at a U.S. Embassy reception last night, Zablocki underlined congressional interest in Spain's entry in the North Atlantic Treaty Organization. "Our interest in Spain's being in NATO comes from our feeling that Spain belongs to the Western family and should play its part in the defense of Europe," he said.

The ruling Union of the Democratic Center party supports NATO entry, and the government has said it will negotiate terms with the alliance after a national debate.

The Socialist and Communist opposition, however, vigorously oppose moves toward NATO, alleging that it would upset the East-West balance. At the embassy reception. Zablocki spoke with senior members of the Socialist Party who, while explaining their opposition to NATO, said they were not opposed to the presence of the four existing U.S. bases in Spain.

Both the Socialist and Communist parties have warmly welcomed the government's decision to attend the Havana conference.

Observers here believe that Spain's decision to attend the Havana conference in the face of conservative criticism and despite the NATO commitment is linked to the plan for Madrid to host the European peace and security conference next year.

Spanish presence at the nonaligned meeting could strengthen its reputation as a neutral country. The previous peace and security conference was held in Belgrade and President Tito, a founder of the nonaligned movement, is a known opponent of its apparent drift towards the Soviet bloc.