IN 1977, when Judge William Webster appeared before the Senate Judiciary Committee at his confirmation hearings as FBI director, he swore under oath that on his watch the bureau would not investigate political dissenters. Webster, reputed to be a man of his word, offered no qualifying national-security clause in his answer.

That is why it is shocking to read that the FBI has been spying on individuals and organizations opposed to President Reagan's Central America policies. After more than five years of active investigation, no one has been charged with a crime. But the FBI has created thousands of new files on innocent individuals.

Apparently, FBI informers already planted inside the Committee in Solidarity with the People of El Salvador (CISPES) and some of the churches offering sanctuary to Central American refugees claimed that the groups were engaged in criminal activities on behalf of a foreign power.

Webster justified opening the secret investigation, which involved 52 of the FBI's field offices, on "national security" grounds. Judge William Sessions, the new bureau director, agrees that the probe was warranted and sees no cause for alarm by members of Congress.

I grew up listening to a radio show called "The FBI in Peace and War." I thought FBI Director J. Edgar Hoover was the epitome of the tough cop, the spy-catcher, the defender of the Constitution. I absorbed those impressions from media eager to foster a positive image of the bureau. I later learned that the FBI had, from its birth, investigated political activity and, from the outset of the Cold War period, had become a full-blown "policy police." It used the loose phrase "national security" to justify spying on and harassing those who disagreed with the foreign policies of the postwar era.

In the early 1970s my colleagues and I often joked about the FBI spying on the liberal think tank where we worked, the Institute for Policy Studies. But we suffered a painful shock of recognition when we learned from some of the FBI's own informers, and then through a law suit, that between 1968 and 1973, that the bureau had ransacked our garbage, reconstructed letters from discarded typewriter ribbons and placed more than 52 informers inside our building.

The operation's code-name was COINTELPRO. In it, beginning in the 1960s, the FBI pursued thousands of individuals and scores of groups who disagreed with government policy on the Vietnam War, on race and on the way the federal budget is allocated. Tens of thousands of citizens became subjects for secret FBI files. FBI agents became involved in robberies, forgeries and other "dirty tricks" against black power and anti-war groups. By the mid-1970s, when these activities were made public, the FBI had a serious image problem. The misdeeds could not be dismissed as aberrations or excessive zeal by individual agents. A former special agent, Robert Wall, offered public testimony on the FBI's illegal harassment of black and anti-war groups and individuals. Internal FBI documents showed that bureau policy was designed to destroy opposition.

When Webster became FBI director 11 years ago, he set out to change the seedy image of the bureau. He agreed to abide by the guidelines written by Attorney General Edward Levi and approved by Congress in 1976. Through the Carter years, FBI special agents appeared to follow them. Agents who had been reduced to watching 80-year-old grandmothers belonging to Yiddish-speaking chapters of the Communist Party were reassigned to catch criminals.

Indeed, during this period, the FBI cracked the most important terrorist case in Washington -- the 1976 assassination of Orlando Letelier, former Chilean ambassador to the United States, and Ronni Moffitt by agents of the Chilean government. Both victims worked at the Institute for Policy Studies, and the FBI agents who broke the case behaved as professional criminal investigators, not political police.

By the late 1970s the bureau appeared to have stopped policing policy opponents and converted to what it was supposed to be: a top-notch investigating organization, more in line with its TV image. New guidelines limited FBI special agents' pursuit of dissenters. Americans could practice free speech without fearing that their words would result in harassment and the creation of a subversive file at FBI headquarters.

Since 1981, however, the "national security" rationale seems to have reentered the FBI vocabulary, creating a loophole in the carefully crafted 1976 guidelines. "Terrorism" became the Reagan policy spearhead, and a supposed "crisis" of a weakened and vulnerable United States produced a new climate. FBI leaders had a convenient pretext: "allegations" of "terrorism" directed by a "foreign power."

The 1987 congressional hearings of the Iran-contra scandals showed that there was plenty of hanky-panky going on inside the administration, a charge that the very targets of the FBI probe had made for years. Indeed, as the facts emerge about U.S. policies toward Iran and Central America, it is clear that those who proposed policies different from the administration's did not merit investigation by a policy police, but rather a serious audience from policy makers.

When Sessions declares that the probe itself was justified but suffered from misdirection, he obfuscates the reality of what happens when the bureau investigates groups that dissent from official policy. As the COINTELPRO experience showed, it is not possible to control the activities of informers, nor curb "excessive zeal" on the part of FBI agents once the notion has been accepted that the targets of the probe are less than good Americans.

The FBI has tacitly defined citizenship as passivity. Yet those who exercise their rights are doing more to perpetuate a meaningful notion of democracy than those who salute when the words "national security" are uttered by an administration official.

Now that FBI misdeeds have been revealed, thanks to a Freedom of Information Act request brought by the Center for Constitutional Rights, the bureau will have to explain to Congress its actions against CISPES and the other targeted groups and individuals. Congress can set an important precedent by asking the new FBI director what he means by national security, and whether it includes the security of U.S. citizens to practice politics that do not coincide with the administration line.

Saul Landau is a senior fellow at the Institute for Policy Studies. His new book, "The Dangerous Doctrine: National Security and U.S. Foreign Policy," will be published in March.