MADRID, Spain (AP) _ In an unprecedented move, Spain's attorney general today filed a criminal complaint against three Basque politicians, accusing them of aiding and abetting terrorism by making threats, his office said.

The Basque separatist group ETA has been trying through attacks to pressure the government into negotiations before the Summer Olympics in Barcelona and Expo '92 this spring in Seville. The criminal complaint apparently reflects government determination to crack down on the separatists' political supporters.

The complaint, which seeks the immediate arrests of the three politicians, does not charge them in connection with any specific attacks, but for remarks construed as threats against government and judicial authorities.

It marks the first time since Spain returned to democracy in 1979 that the country's highest prosecutor has targeted members of a legal political party for statements alone.

The three accused are members of the national executive committee of the radical Basque Herri Batasuna, a coalition generally viewed as the political arm of ETA.

The complaint came hours after a car bomb blast in the Mediterranean city of Murcia killed a policeman, and four days after a bomb explosion in central Madrid killed five people. Authorities blamed ETA for both attacks.

On Thursday, following the Madrid bombing, Socialist Prime Minister Felipe Gonzalez said the government would ''tighten the judicial noose around those who are running loose and feel they can threaten people.''

A judge must determine whether actual criminal charges will be filed as a result of the complaint. Charges of threatening the government carry penalties of more than 12 years.

Named in the complaint by Attorney General Leopoldo Torres are Jon Cruz Idigoras, a deputy in the lower house of the Spanish Parliament; Floren Aoiz, deputy in the regional parliament of Navarre; and Adolfo Araiz.

Torres' complaint says the three threatened the central government, the regional Basque government, an investigating judge and a regional Basque official.

The attorney general told reporters the threatening statements were made at a news conference the party held Jan. 31 in Pamplona. Party members objected to the arrests of several other party members in connection with the alleged collection of a ''revolutionary tax'' in ETA's name in the Basque region.

Idigoras, Aoiz and Araiz were all present, and Idigoras said the party would not ''sit by with its arms crossed in face of these attacks and detentions.''

News reports said Torres was also planning to seek the arrests of Patxi Zabaleta, a member of the party's national executive committee, and another party member, Jose Maria Olarra.

In the last general election in 1989, voters elected four members of Herri Batasuna as deputies to the 350-seat lower house of Parliament and three to the senate. The party holds 13 of the 75 seats in the Basque regional parliament and six of the 50 seats in the Navarre assembly.

However, none of the national deputies or senators has taken his seat because they refuse to swear allegiance to the Spanish Constitution.

ETA has claimed responsibility for killing 700 people since 1968, when it began its campaign to win independence for the northern, three-province Basque region. ETA is a Basque-language acronym for Basque Homeland and Liberty.

Murcia, the site of today's bomb attack, is a provincial capital 215 miles southeast of Madrid, between the Mediterranean ports of Almeria and Alicante.