The federal Bureau of Prisons and the American Civil Liberties Union's national prison project yesterday offered sharply differing appraisals of a confrontation between gurads and inmates last year at the federal prison in Lewisburg, Pa.

The ACLU group, announcing a civil suit on behalf of 38 federal prisoners, charged that Lewisburg guards brandishing ax handles attacked and beat two groups of prisoners who were bound with chains and handcuffs.

"We have not seen this kind of incident since the Attica assault," said Alvin J. Bronstein, the head of the ACLU prison project. "It was a planned assault on inmates who had no way to protect themselves. And it was unprovoked."

The Bureau of Prisons responded by making public its own internal investigation. That report said "no more force was used than necessary... no inmate was kicked, beaten, or assaulted." The bureau added that its investigation was hampered because the ACLU advised the prisoners involved not to discuss the incident with the bureau's investigators. The ACLU said this advice was given because federal officials would no permit lawyers to be present during the interviews.

Both sides agree on some aspects of the case. On April 14, 1978, two busloads of prisoners arrived at Lewisburg. They were met by a large number of guards in riot gear armed with nightsticks and ax handles. The prisoners were taken to holding rooms to be searched for weapons before being sent to cells.

The Bureau of Prisons said this unusual reception procedure was necessary because Lewisburg officials had evidence that the prisoners, who were being transferred from another prison, had weapons on the bus and keys to unlock their shackles.

The bureau said the officers achieved their objective of "intimidating" the prisoners merely by turning out in force as the buses arrived. It said there was no attack on the inmates, but that three of them suffered "minor abrasions" in the unloading process.

The ACLU group said the guards beat the prisoners en masse as they stepped off the bus and individually thereafter. Bronstein said this was part of a pattern of "systematic" brutality against inmates at Lewisburg.

The suit seeks $200,000 apiece in damages for five of the 38 inmates and $150,000 for each of the others, for a total of $5,950,000.