A top FBI official told the Senate intelligence committee yesterday that no one in the White House prompted or encouraged an FBI investigation of a group that was publicly opposed to Reagan administration policy toward El Salvador.

Testifying under oath before a panel that was openly skeptical at times, Oliver B. Revell, executive assistant director of investigation, offered a spirited defense of the FBI's probe of the Committee in Solidarity with the People of El Salvador (CISPES), rejecting any suggestion that the investigation's purpose was political spying.

"Nothing could be further from the truth," Revell said. He said the Federal Bureau of Investigation has probed "a number of groups on the other side of the political spectrum."

The FBI's conduct in the CISPES case is under scrutiny by two congressional committees and by FBI inspectors. Yesterday's hearing was prompted by the release Jan. 27 of 1,200 pages of FBI files, which showed that field agents had collected the names of church groups, college student organizations and unions that the bureau said were involved with CISPES. Agents also had photographed participants in public gatherings involving the group.

Acknowledging that "we may have seen some wavering over that line" between a legitimate investigation and political spying, Revell testified that there was a legal basis for the probe: circumstantial evidence in 1983 -- which the FBI never documented -- that CISPES members were sending money to the Farabundo Marti Liberation Front (FMLN), the main insurgent group trying to overthrow the government of El Salvador.

The FBI classified the FMLN as a terrorist group in El Salvador and suspected that FMLN members were involved with bombings in Washington in 1983-84. Because of CISPES' professed support for the FMLN, Revell asserted, it was important to gather intelligence about the two organizations.

"Although our two-year counterterrorism investigation failed to lead to indictments of CISPES members, we did find indications that some CISPES members were at least discussing and planning violence," he testified.

He listed several examples, based on information he said was provided by FBI sources: One CISPES member was "tasked to determine response times of emergency services in a major American city"; another member "stated he had developed a system to shut down a public utility in a major midwestern city, as well as plans to violently disrupt the 1984 Republican convention." Revell did not elaborate.

After Revell's prepared statement, some panel members questioned whether the FBI's decision to conduct the probe under foreign counterintelligence guidelines gave the bureau too much latitude. "There is a real straining to justify your investigation," said Sen. Arlen Specter (R-Pa.), asserting that the agency should have proceeded "on tangible evidence," not unconfirmed allegations.

In June 1985, the Justice Department determined during a required annual review that the investigation had not produced enough evidence to warrant its continuation. It was closed two weeks later.

Revell said the bureau had acted properly. "Those in the FBI responsible for authorizing the counterterrorism investigation believed information, which had been brought to their attention, that CISPES was acting as the agent of {the FMLN} . . . . The predication for the investigation was based upon what was known at the time."

But Sen. William S. Cohen (R-Maine), vice chairman of the Select Committee on Intelligence, said the effect of the CISPES investigation was to impinge on the rights of U.S. citizens: "Whatever the rationale . . . it resulted in the FBI subjecting many Americans and U.S. organizations to various types of scrutiny without any basis other than the exercise of their First Amendment rights."

Among the 1,200 pages of documents released so far are several that show FBI headquarters attempting to curtail the flow of information about CISPES' political activity.

A July 26, 1984, teletype sent to field offices said: "Political activities or political lobbying by CISPES . . . are not, repeat not, targets of this investigation and should not be monitored." Some field offices, however, continued to supply such information. On Oct. 17, 1984, a second teletype laid out specific guidelines for gathering information.