Proceedings were halted when the prosecutor, Marco Aurelio Nascimento, walked out of court in protest against the judge's refusal to send the acquittals to appeal.

The police commanders were the first of 150 officers to be tried for the killings, which happened when police fired at protesters calling for land reform.

But after a 40 hour trial, on Thursday the seven-member jury found there was insufficent evidence to convict the men of leading the killings.

The judge said the trial of the remaining officers should carry on immediately.

'Feeling of impunity'

Prosecutors had said that because of the acquittal of the three senior officers they had no chance of convicting the more junior men.

Human rights and church groups condemned the acquittals, which the President, Fernando Henrique Cardoso, said added to the feeling of impunity in Brazil.

President Cardoso said that "speaking as a citizen" the acquittal was "lamentable", adding that "a feeling of impunity is what prevents the consolidation of democracy."

Human rights groups have backed Mr Nascimento's decision to walk out.

Trial suspended

The prosecution's abandonment of the courtroom forced the judge to suspend the trial indefinitely.

The trial has the largest number of defendants in Brazilian legal history, but the defendants are being judged in small groups, with the officers who commanded the operation appearing in the dock first.

The fact that the first three officers have been found not guilty came as a blow to legal rights groups in Brazil who campaigned for the trial.

Correspondents say the killings have focused national and international attention on the unequal land distribution in Brazil, where the poorest 40% of the people own just 1% of the land.