FBI Director William S. Sessions denied yesterday that the bureau "mounted a massive surveillance campaign against American citizens opposed to the administration's Central American policy," but he acknowledged that the probe of a liberal group "was not properly directed" in all instances.

Speaking to reporters at FBI headquarters three months after becoming director, Sessions offered the bureau's first detailed explanation of documents released last week by the Center for Constitutional Rights, a New York lawyers' group.

The documents, obtained under the Freedom of Information Act, showed that the Federal Bureau of Investigation had extensively investigated the Committee in Solidarity with the People of El Salvador (CISPES).

During their investigation, agents photographed protesters at peace rallies and recorded their license-plate numbers; investigated nuns, unions members and college students, and checked up on church forums and Knights of Columbus dinners, according to the documents.

Although Sessions said the complete file "would provide a much clearer picture of our actions," the heavily edited documents portray agents reporting and sometimes commenting on political matters.

"In his writings, you can see a mind totally sold to the Marxist- Leninist philosophy," the Dallas office said about an Episcopal priest, adding that his program on Central American affairs featured " 'Liberation Theology,' anti-American rhetoric and a complete distorted view of the role of the United States in world affairs."

Sessions said the probe was prompted by tips that key CISPES members "were involved in covertly furnishing funds and material to a foreign terrorist organization," the FMLN, a leftist opposition group in El Salvador.

He said FBI agents conducted only "limited investigation" of other groups "to round out or develop information on the scope of activities and influence of CISPES."

Sessions, who is conducting an internal review of the investigation at the request of President Reagan and Attorney General Edwin Meese III, testified yesterday at a closed meeting of the Senate Select Committee on Intelligence.

Chairman David L. Boren (D-Okla.) said the panel had asked for "all unedited documents relating to the investigation of domestic groups" and had told Sessions "the seriousness with which it views allegations of inappropriate activity" in such surveillance.

Sen. Bill Bradley (D-N.J.) said after the meeting that he had told Sessions, "The FBI lost all credibility in the 1970s, and it is losing credibility again as a result of these reports in the press, which, if true, are utterly outrageous, unacceptable and reprehensible."

The FBI probe, which occurred under Sessions' predecessor, William H. Webster, was closed in June 1985 when the Justice Department concluded "that CISPES was involved in political activities involving First Amendment rights and not international terrorism," Sessions said.

Sessions said field agents had been "explicitly instructed . . . that the investigation must not interfere with the exercise of First Amendment rights of those CISPES members who politically opposed U.S. policy in Central America."

He said, however, that field agents did not always follow those directions. "I think you would have to say in some instances, had the matter been reviewed at that time and sensed at that time, that there might have been . . . different" directions, he said.

Asked about a memorandum from the FBI's New Orleans office stating that it was "imperative . . . to formulate some plan of attack against CISPES and specifically against individuals . . . who defiantly display their contempt for the U.S. government," Sessions said the investigation "was not out of control but, as to that particular facet, it was not properly directed."

"I would have hoped that it would have been directed differently," he said. "But that is, of course, very difficult even in a relatively small investigation."

Later, an FBI statement expanded on Sessions' comments. "The information in that . . . is something that an agent would say," it said. "However, the document contained intemperate language and recommended a strategy . . . not considered or adopted by FBI headquarters . . . . "

Sessions defended such practices as taking photographs and license-plate numbers at rallies. "To gather information is essential. And, until you know what involvement is in the leadership and what involvement is in the key members, you have no right to simply pull back because it is in some extent intrusive in the American public's way of life," he said.

Although Sessions steered away from criticizing Webster, now director of central intelligence, he said, "I think it's very important that leadership be exercised to ensure that investigation is focused correctly and, if it is not correctly focused . . . that there be a direction reestablished."

Bill Baker, Webster's spokesman at the FBI and now at the CIA, said he has not spoken to Webster "about whether he's satisfied or not" with conduct of the investigation.

"I don't think you're going to find any widespread civil-liberty transgressions in this investigation," Baker said. "You're hearing a lot about it but think about the group you're hearing it from and the timing of it," he said, referring to today's scheduled House vote on continued aid to the Nicaraguan contras.

Margaret Ratner, an attorney with the Center for Constitutional Rights, disputed Sessions on the limited focus of the investigation, saying that the FBI watched more than 200 groups supposedly linked to CISPES.

"The reputation of many teachers, students, trade unionists, ministers, sisters and other church people were affected because the FBI went through their home communities asking questions about them in the context of what the agents said was a 'terrorist investigation,' " she said.

In a statement, CISPES said nothing that Sessions had said "justifies the nationwide probe that included the use of paid informants, surveillance and harassment of members."

Sessions declined to comment specifically on what he described as "extremely serious allegations" by Donald Rochon, a black FBI agent who has said he was the victim of racism by fellow agents in Omaha and Chicago. Rochon has sued the bureau, which is conducting a criminal investigation of his complaints.

However, Sessions said he thinks that Rochon's case was "an isolated incident" and added that he is committed to bringing more minorities and women into the bureau.

Staff writer Howard Kurtz contributed to this report.