The one legal opposition political party in Brazil today defied the authoritarian military regime by blocking a government-sponsored constitutional amendment in Congress.

There was widespread speculation that the regime would react by closing Congress or firing large numbers of opposition congressmen.

This most serious confrontion in recent memory between the armed forces and civilian politicians could set back an announced government plan to return Brazil to democracy gradually.

The opposition Brazilian Democratic Movement, known as the MDB from its initials in Portuguese, holds 42 per cent of the seats in the 429 member, two-house Congress. It voted unanimously against the amendment, preventing the two-thirds majority needed to pass.

Many MDB congressmen who voted no had been freely elected in 1974, when President Ernesto Geisel carried out his promise to hold open elections despite strong opposition within the armed forces.

Geisel, a retired army general, has dictatorial powers but has pledged to use them sparingly, if politicians go along with his ground rules for gradual liberalization, which are some times hazy. One of these rules is not to question the basic authority of the armed forces.

Thursday is the 13th anniversary of the coup that put the miliitary in power here. Geisel has an important speech to make at an army base, and according to observers, pressure within the armed forces will oblige him to react strongely to the opposition party's veto. The congressional defeat is being interpreted as a direct affront to the armed forces, leading political figures say privately . . .

The confrontation in Brazilia concerned a complicated constitutional amendment to restructure the judicary branch. There have been many complaints about slow, inefficient court proceedings.

The MDB said, however, that the government's proposed amendment dealt only with minor technical issues and did not protect judges against summary removal by the executive branch. Nor did it strike down the current ban on habeas corpus for defendents in political subversion cases.

These provisions are included in a sweeping dictatorial decree known as Institutional Act No. 5. To limit these two powers in the judiciary amendment would challenge the basic premise that the armed forces control the government.

The MDB leader in the Brazilian house of representatives, Rep. Alencar Furtado, declared earlier that the opposition had no choice but to stand up to the government and veto the amendment "because the purpose of our party is to gain a return to democracy." He added: "The MDB refuses to be the regime's errand boy."

Another MDB congressman, Rep. Pancredo Nerves, argued, however, that the present state of affairs was "too delicate" for a direct challenge to the regime.