Warning against the notion that "it can't happen here," Sen. Jeremiah Denton (R-Ala.) opened hearings yesterday on the "new threat" of terrorism that, he said, is endangering freedom in the United States and elsewhere.

A succession of witnesses, led by former CIA director William E. Colby, blamed the Kremlin and a number of its allies for training, supporting and equipping terrorists on a far-flung international scale, but agreed that there was no "mastermind" directing the violence.

"I think there is a feeling that there is a central war room with flashing lights," Colby told Denton's subcommittee. "There is no central war room." The Soviet Union "did provide the instruments," but neither the Russians nor any other government, he said, is "directing the orchestra."

The inquiry, the first in a sporadic round of sessions to be held in coming weeks, produced instant controversy and echoes of the past. Yesterday morning, a small group of protesters showed up as the Ad Hoc Committee Against the SST (Subcommittee on Security and Terrorism) to warn against a return to the red-baiting days of the 1940s and '50s. Disapproving press releases abounded.

"The most worrisome factor about the hearings today is that they have not been called to consider pending legislation," Americans for Democratic Action national director Leon Shull said in one statement. ". . . this subcommittee has no plans for legislation. It appears to be seeking only a platform to make its inflammatory charges about the nature of terrorism in the world today."

The ADA charged that "such activity can prove a great threat to civil rights in this country."

Denton, a Navy veteran who spent nearly eight years as a prisoner of war in North Vietnam, allowed in his opening statement that a primary purpose of the hearing was "to raise public consciousness and support" but he clearly did not see it as a threat to civil rights.

"If we continue to ignore the threat or define it away by academic gymnastics," the Alabama Republican declared, "the sand in which we bury our heads will eventually bury our nation."

The threat in any case will appear more formidable in fresh statistics that the CIA is preparing. Denton confirmed that the agency has "redefined" terrorism in a much broader fashion than it did in previous years. One result will be to show thousands of more terrorist incidents around the world over the last 13 years than had been reported.

Under the CIA's new definition, "terrorism is the threat or use of violence for political symbolic effect that is aimed at achieving a psychological impact on target groups wider than its immediate victims." It can also apparently include activities within a single country that stem from insurrectionary or military operations.

Under the old definition, there had to be a "credible threat" of violence, if not actual violence, and the terrorism had to "transcend national boundaries" in some discernable fashion.

Testifying after Colby was journalist Claire Sterling, author of The Terror Network, a book that describes Soviet and surrogate support for terrorists of all persuasions, including Irish extremists and Spanish Basques. She said such collaboration began roughly around 1968, but suggested the Soviet contribution could best be described as "a do-it-yourself kit for terrorist warfare" that might destabilize Western societies.

Sen. Patrick Leahy (D-Vt.) questioned her about the book's charges that Western intelligence agencies have long known but said nothing about the Soviet involvement.

"Have the CIA and the FBI been bought?" he asked after noting that she had written "the fix is in."

"Well, I don't know about the FBI," Sterling replied. She added that she wasn't talking about just the CIA, but about "all the Western governments" that have been under attack by terrorists. Those governments, she said, have been "extremely reluctant to point a finger at the Soviet Union," perhaps to preserve the spirit of detente perhaps for other political reasons.

"I don't have a plotter's mentality," she said. "The fix is political."

Sterling, who has lived in Italy for 30 years, declined to suggest what should be done in America. It was widely agreed at the hearing that, as another witness, Washington Quarterly editor Michael Ledeen, put it, "at the moment, domestically, there is no problem to cope with."

Security at the hearing was, nonetheless, tight. The public had to enter through a metal detector. Plainclothes security officers dotted the room, although subcommittee chief counsel Joel Lisker, who asked for their presence, told reporters that no threats had been received.

During the afternoon session, Denton announced that he had received a call from a Huntsville, Ala., television station, asking him if he was wearing a bulletproof vest. He then unbuttoned his shirt a bit and announced that "the answer is no."

Another main topic at the hearing was the issue of "disinformation," on which, Denton said, the Soviet KGB expends much effort, especially in "recruitment of Western journalists."

The primary witness on the issue, former Newsweek correspondent Arnaud de Borchgrave, said he became interested in the subject in the late 1960s when he learned that a friend and colleague in France was on the KGB payroll.

"He had been sexually compromised through his wife," de Borchgrave said. He did not elaborate.

Citing a number of leading scientists from Andrei Sakharov to Fred Hoyle, de Borchgrave also suggested that "the Soviet Union today is playing a covert role in the antinuclear lobby."

He charged that "there is a direct link between the World Peace Council, a well-known Soviet front organization, and anitnuclear lobbies both in the U.S. and West Europe." De Borchgrave added that the peace council's U.S. branch is "affiliated with MFS -- Mobilization for Survival -- which is a leading umbrella organization for antinuclear groups."

"Interestingly enough," de Borchgrave continued, "MFS has now linked the antinuclear protest to unilateral disarmament advocates, New Left activists and some ecologists." But he said current FBI guidelines make it difficult for the bureau to monitor "the very groups and individuals that the KGB hopes to manipulate or recruit."

MFS, a Philadelphia-based organization, issued a statement last evening denouncing suggestions of external domination as "total fabrication" and protesting that "the kind of guilt by association tactics employed by Mr. de Borchgrave can only be said to be reminiscent of the repression and intimidation of the McCarthy era."

Throughout the hearing, Denton professed his hopes of a "rapprochement" between the U.S. press and government, and repeatedly expressed his chagrin over the news media's performance during the Vietnam war. At one point, Colby alluded to the CIA's finding that the antiwar movement was "an indigenous movement" and not dependent on foreign support.