ATLANTA, NOV. 30 -- A key Cuban-American hostage mediator and a government spokesman said today that an "aggressive minority" of hard-core criminals is intimidating the majority of Cuban detainees into prolonging the takeover of the federal prison here.

The 90 remaining hostages are divided between the two groups.

Jorge Mas Canosa, President Reagan's former Florida campaign manager and a key outside mediator with the 1,118 inmates holding the federal prison, said the hard-core group torpedoed an agreement that he and two other prominent Cuban-Americans thought they had reached with inmate representatives Sunday night. Under that deal, the inmates would have released 50 hostages and would have been allowed to hold a news conference.

Earlier Sunday, Cuban detainees in Oakdale, La., reached an agreement with federal negotiators -- throwing down their arms and releasing the remaining 26 hostages.

"The radicals want everything," Mas Canosa said of the Atlanta situation.

A Justice Department official who asked not to be identified said that some of the demands of the detainees here are harsher than those in Oakdale, which may mean the siege will continue for several more days.

He said their key demand is immediate citizenship, rather than the quick hearings on their status promised the Oakdale inmates. Inmates here also want guarantees that there will be no reprisals and that they will be allowed, like the Oakdale inmates, to seek asylum in any third country that would accept them if they are ordered deported. He said they also demanded direct contact with Oakdale detainees, apparently to determine whether government promises were kept.

There was a brief negotiating session tonight, but officials declined to say who participated or what was discussed.

Mas Canosa said that the hostages are safe and that he had cautioned the inmates to avoid bloodshed "at any cost." A majority of the inmates seem "calm and prudent," he said.

Justice Department spokesman Patrick S. Korten said the detainees made videotapes of the hostages Sunday night, at the request of authorities, and that the tapes "demonstrate clearly that a majority of the hostages are safe and in apparently good health." He said 70 hostages appeared on the tapes, "and we have verified the safety of all of the rest by other means."

Korten repeated the government's promise not to retake the prison by force as long as the hostages remain safe, but "if any of the hostages are harmed, then we reserve the right to take whatever action is necessary."

Korten's statement appeared to be an effort to encourage the larger group of inmates to assert themselves. The statement was unusually frank and detailed in the midst of a government public relations effort that has mostly attempted to control dissemination of information about what is happening inside.

Korten said it is "our impression" that a majority of inmates -- including the three who have done most of the talking with federal negotiators -- want to reach an agreement. "Unfortunately, a small but aggressive minority appears to be able to intimidate this majority into dragging out the incident and avoiding a settlement. We are prepared to be as patient as necessary as we await a decision by the Cuban detainees to bring the incident to an end and settle it on a fair and equitable basis."

He said he does not know how many are in the minority group, but that "it could be 100 or more." He revealed little else about the group other than to say that the minority was responsible for blocking a tentative agreement to release 50 hostages on Thanksgiving as well as several less sweeping initiatives.

Mas Canosa said last night that he and the other two Cuban exiles brought in to mediate "felt that we had a deal. We were extremely close." But by this morning there was no deal. The other negotiators are Roberto Martin Perez and Armando Valladares, both former prisoners of Cuban leader Fidel Castro.

He said the three met Sunday night with three representatives from the moderate group and two from the radical group to explain the agreement that had just been reached with inmates in Oakdale. The radicals, he said, asked them to speak to a larger group of perhaps 200 or 300 inmates, but few questions were asked about the Oakdale agreement.

As the hostage drama entered its second week, there was no indication that the Oakdale agreement had any effect on the situation here.

"As a matter of fact, they {the radicals} did not pay too much attention to the release of the hostages in Oakdale," Mas Canosa said.

The only indication today that the inmates had taken any note of Oakdale were signs that appeared on a prison rooftop late in the day saying, "We want to talk to Bishop San Roman," referring to Cuban-born Bishop Agustin A. Roman of Miami, who played a key role in the Oakdale settlement. Another sign offered a list of those the inmates want to see -- Rep. John Lewis (D-Ga.), activist attorney Gary Leshaw, an otherwise unidentified "Rev. Gregory," Atlanta City Councilman Hosea Williams, U.S. District Court Judge Marvin Shoob and the news media.

Korten said there are no plans to bring Roman to the scene, although that remains a possibility.

Korten said the Oakdale agreement may eventually have some impact on inmates here, "but this is not the sort of case where you want to count your chickens. This is a different kind of group of detainees." He said Oakdale housed minimum-security prisoners, while many of the inmates here are hard-core criminals.

Nevertheless, the Cubans last night delivered to authorities the most dangerous prisoner in the penitentiary -- an American inmate, Thomas E. (Berserko) Silverstein, who is serving three life sentences for murder.

The inmates brought Silverstein in shackles and handcuffs to federal authorities. An obviously pleased Korten said at a late-night news briefing that the inmates were fed up with Silverstein and turning him over indicates they are concerned with the safety of the hostages.

There had been concern that Silverstein, who was kept in permanent solitary, might harm the hostages.

It was difficult to gauge the mood inside the prison today. There was less two-way radio chatter and loudspeaker sloganeering than there was Saturday night and Sunday morning, and fewer family members stood across the street from the imposing gray building to scream and shout greetings to inmates atop a building inside the grounds. The gloomy weather -- following a mild and sunny Sunday -- also apparently put a damper on inmates' spirits. Officials said the prison heating system was destroyed on the first day of the takeover and cannot be repaired.