In a surprise action that puts a formal end to nearly a decade of strict and sweeping government curbs on the press here. Brazil's military rulers have lifted prior censorship from the last three publications still subject to restraint.

The announcement was made to editors of the newspapers Tribuna da Imprensa and O Sao Paulo and the magazine Movimento through an anonymous phone call from censors late Thursday and confirmed Friday by a spokesman for Brazilian President Ernesto Geisel. According to the spokesman, the measure is part of Geisel's effort to carry out a program of "gradual and progessive political opening."

The order does not extend to radio or television, the main sources of news in this nation where an estimated 25 percent of the population is still illiterate. In addition, newspapers here have noted that the government action represents "a concession, not a right" and that new interventions could come "at any moment."

The editor of Tribuna Da Imprensa, a Rio daily whose circulation has dropped from 100,000 to 10,000 since prior censorship was introduced in 1968, responded by announcing he would sue government censors for the paper's losses.

Editors at Movimento, a weekly with socialist tendencies said they would immediately begin publishing the 3,093 articles and 3,162 photographs censors had vetoed. O Sao Paulo is a newspaper of the Catholic Church.

Gen. Geisel took power in 1974 promising to initiate a policy of domestic "detente" and political liberalization that would lead this nation of 115 million back to democracy. But his rule has been marked by frequent reliance on the arbitrary powers that the military has accumulated since seizing power in a coup here in 1964.

Now, however, in the waning months of his five-year term, Geisel is showing signs of being willing to loosen his grip on the nation. The ending of prior censorship, which has been hailed by the press as "an important step forward," is one of several recent policy decisions that are seen here as signalling the advent of a more open political climate.

Last month, more than 50,000 auto and other factory workers in the industrial belt around Sao Paulo went on strike demanding higher wages and the right to negotiate directly with employers without government interference. It was the first major labor action here in 10 years, and was conducted in defiance of a law that virtually forbids strikes.

Despite a judicial decision declaring the Sao Paulo strike illegal, the government said it viewed the workers' behaviour as "natural" and took no steps to send in troops or police, as it had done in 1968. In the end, it allowed employers to grant large salary increases, in direct contravention of the national wage law that is the keystone of the economic strategy which has turned Brazil into the world's 10 largest economic year.

In addition, Geisel's office announced yesterday that it was lifting the order of banishment against Ricardo Zarattini, allegedly a member of a terrorist group active here in the late 1960s. Zarrattini was one of the 15 political prisoners released and flown into exile in 1969 after the then U.S. ambassador, Charles Elbrick, was kidnapped by urban guerrillas in Rio.

Zarattini, who has returned to Brazil and is in custody here, will have to face charges. But the government action, the first of its type, is seen by observers here as paving the way for the return of others of the 121 Brazilians exiled by government order for "subversive" political activity.

These actions come after two recent speeches in which Geisel promised widespread political reforms before the end of his term next March. According to public statements by leaders of ARENA, the official government party, the reform package will include revocation of Institutional Act Number Five, which gives Brazilian presidents dictatorial powers, and its replacement by constitutional "safeguards."

Observers here attribute Geisel's sudden show of tolerance in part to a desire to improve the image - and chances - of ARENA in elections scheduled for this fall. Polls show that the government party will be decisively defeated by the sole legal opposition party, the Brazilian Democratic Movement. That would complicate the situation for Geisel's handpicked successor, Gen. Joao Baptista Figueiredo.