An estimated 40,000 flag-waving supporters of Spain's Communist Party greeted party president Dolores Ibarruri (La Pasionaria) in the Basque city of Bilbao today. Five Basque political prisoners, meanwhile, were released from jail and flown to exile in Belgium.

The two events signaled Premier Adolfo Suarez' intent to try to reduce political tensions before the June 15 parliamenting elections, Spain's first free vote in 41 years. The campaign officially begins Tuesday.

Another 19 Basques jailed for separatist guerrilla activities against the late dictator Francisco Franco are expected to be exiled in the next few days, according to their lawyers. All but one are members of ETA, the Marxist-Leninist Basque liberation movement that is fighting for the region's independence from Spain.

The chance of success of Suarez' moves to calm the Basques remains unclear.

ETA rejected the government's exile formula last night and annouced that it is resuming "armed intervention until the demands of the Basque people are truly put into effect." The underground also condemned Basque political parties - including the Communists - for accepting the government's offer to exile the prisoners.

Meanwhile, the Communist Party, which has assumed the moderate Eurocommunist line, was on its best behavior for La Pasionaria's first public appearance since her return nine days ago from 33 years of exile in Moscow. The 1936-39 civil war figure was showered with red carnations at the meeting.

With tears in her eyes, Ibarruri, 81, praised the Soviet Union and recalled her native Basque homeland in a brief speech.The meeting was described as the biggest political gathering in the restive Basque region in decades.

Later, the party's secretary-general, Santiago Carrillo, said that Ibarruri had suffered "an emotional shock" during the meeting and was resting. He said that he hoped she would recover in time to run in the election.

Carrillo and Ibarruri reportedly have doctrinal differences over direction from the Soviet Union. There is little doubt, however, that Carrillo dominates the party.

Carrillo appeared on the state-controlled teveision network Friday night for a lengthy explanation of the party's doctrine and what he means by "Eurocommunism."

A television political talk by Carrillo would have been unthinkable only two months ago, as was the appearance of La Pasionaria at a massive rally filled with Communists waving Communist and Basque flags.

The arrival of the Communist leadership in Bilbao did not stimulate any reaction even the kidnaping of Javier de Ybarra, a rich conservative industrialist. The abduction - attributed to ETA - took place before the government's weekend decision to offer exile to Basque political prisoners.

The ruling on Basque priosners is one of the most controversial by Premier Suarez in his 10 months in office.

Basques said that the premier had been considering, the exile solution for several months, but had failed to act on it because he was reluctant to arouse opposition among the military and among the still powerful right.

Rightists throughout Spain consider Basque self-rule and separatism as treason.