Vice President Walter Mondale arrived in Spain today and strongly praised the 18-month-old government of King Juan Carlos for its "love and support of democracy and human liberty," as domestic critics intensified calls for more political freedom.

Mondale flew here from Lisbon for a one-day visit as bitter civil srtife and strong police reponse seemed to be abating in the four Basque provinces of northern Spain. Calm was reported in the region, where three persons have died from police bullets in the past week.

In sharp contrast to Mondale's unqualified praise and encouragement, the archbishop of Pamplone, Jose Mendez, in a statement issued late last night, attributed the latest round of upheaval in the Basque country to "the lack of the most elemental human rights: the right to assembly, expression and association."

The unrest has been taking place in Basque cities hundreds of miles north of the Spanish capital, but Madrid was also set on edge today as a powerful bomb planted in the American Embassy's cultural center exploded at about 4 a.m., six hours before Mondale arrived.

Callers claiming to represent a leftist terror organization known as Grapo telephoned Spanish newspaper, asserting that they had set the bomb to protest "Yankee imperialism," the king's policies and the "domesticated" political opposition working within the system.

The bomb was evidently concealed in a classroom used to teach English to Spanish students. The cultural center is in central Madrid, across town from the embassy compound.

A nightwatchman was stunned by the blast but there were no serious injuries. Damage to the classroom was described as substantial.

Mondale is scheduled to leave Madrid Wenesday morning for Vienna, where he is to meet South African Prime Minister John Vorster for talks on ending the guerrilla wars in Rhodesia and Namibia (Southwest Africa) and to express the Carter administration's strong concern about human rights include inside South Africa.

The still developing U.S. stance on southern Africa, which will depend parly on Vorster's reaction to the Vienna talks, is emerging as a central part of the Carter administration's human rights campaign, which appears to be entering a new phase after initially concentrating on the Soviet Union and Eastern Europe.

The stops in Lisbon and Madrid were also used to emphasize the new American commitment. In both Portugal and Spain, Mondale praised the governments for coming out of decades of right-wing dictatorships, supported by previous American adminstrations, and moving toward democratic parliamentary systems.

Portugal has held three free national elections since the 1974 coup that toppled the dictatorship. The leftist military officers who staged the coup lost out to Premier Mario Sores' Socialist Party Cabinet.

In Spain, the results have been far more mixed since the death of Francisco Franco, who appointed Juan Carlos as his successor. Mondale aides said today that "the principal focus" of the talks with Juan Carlos and Prime Minister Adolfo Suarez had been Mondale's effort to "encourage" them to continue moving toward democracy.

Suarez has set up the framework for the first free parliamentary elections in Spain since 1936. They are to be held June 15, and Suarez is leading a centrist alliance against the challenges from Franco supporters on the right and Communists and Socialists on the left.

Although Mondale insisted to reporters that his trip here was not timed to boost Suarez' campaign, Spanish political analysts close to the prime minister said the visit and lavish praise Mondale voiced on his arrival would unquestionably be viewed that way.

"Your nation and its leadership have demonstrated to the world that the forces of democracy are not on the decline but are on the offensive," Mondale said at the airport.

He then had a two-hour talk and a luncheon with the prime minister.

"You and President Carter got along very, very well," Mondale told Suarez, referring to the Spannish leader's call on Carter in Washington last month.

Since returning from Washington , Suarez has been faced with more public demands for full political freedoms, and his government has authorized riot police and civil guards to deal harshly with demonstrators.

Public demonstrations are illegal in Spain. Police have clubbed and beaten demonstrators and have fired smoke grenades, rubber bullets an d in some cases live ammunition, at protestors and bystanders in Madrid and the Basquet country this month.