One district attorney called the case ''the most frightening and bizarre criminal conspiracy I have ever seen.'' 'Who's Burning Boston?'

Mr. Higgins held up copies of Boston magazines of the period that he said published cover articles asking the question, ''Who's burning Boston?''

''We're here today because we think these charges are a step toward answering that question,'' Mr. Higgins said.

The indictments allege perjury, obstruction of justice, threatening of witnesses and destruction of evidence, as well as conspiracy. Affidavits presented at a Federal bail hearing this afternoon further allege that one of the defendants threatened to kill the supervisor in the Bureau of Alcohol, Tobacco and Firearms who led the investigation.

Other defendants, the affidavits allege, urged a Boston police officer who had already pleaded guilty in the case to flee to Canada before sentencing. He was subsequently sentenced to state and Federal prison terms and is expected to be a witness in the Federal trials. Clubs of 'Fire Buffs'

At the hearing for the men arrested in the Boston area, a Federal magistrate ordered Donald Stackpole, Gregg M. Bemis and Wayne S. Sanden held without bail, and set bail for Ray J. Norton Jr. and Joseph M. Gorman at $50,000 and $25,000, respectively.

Mr. Weld said the group had had its genesis in a little-known but organized world of people fascinated by fires. ''My understanding is that there are certain clubs in Boston where fire buffs are wont to hang out,'' he said.