DALLAS (AP) _ A man suing FBI officials in a pay dispute says agents broke into offices of groups opposed to the Reagan administration's Central America policy and stole documents while he worked as an agency informant.

Frank Varelli said Wednesday the FBI investigation began in 1981, and that he had infiltrated the Dallas chapter of the U.S. Committee in Solidarity with the People of El Salvador (CISPES) on FBI orders.

Oliver B. Revell, executive assistant director of the FBI, said Varelli ''was never an FBI agent. He did have a relationship with the FBI in providing information. That was a voluntary relationship.''

Revell said the bureau is conducting ''a criminal investigation on a former FBI agent who was involved with'' Varelli. Speaking on a C-SPAN television interview Wednesday night, Revell said the former agent, whom he did not name, resigned during an FBI administrative inquiry into allegations of improper conduct on the agent's part.

Revell denied the FBI spied on anyone but said it did ''undertake certain investigative activities based on information that came to us through legitimate means. It was within the guidelines established by the attorney general. It was reported to the Justice Department and reviewed by the Justice Department.''

He added that he expected the criminal investigation of the former agent to establish whether Varelli's accusations were accurate or not.

''Whatever documents in 1982-1983 that I was not able to obtain legally, the bureau designated, on a regular basis, two agents to come to the Bethany House and go in and obtain those documents,'' Varelli told about 30 CISPES members here.

Bethany House is a Catholic religious community in Dallas.

''The documents had more to do with organizational structure and membership,'' he said.

The 37-year-old Varelli, who moved to this country 18 years ago, also said he went to his native El Salvador on FBI orders to talk with that country's national guard.

He said he gave guard officials information on traveling Americans, Salvadorans and Salvadoran deportees, and that some of the deportees may have been killed by right-wing death squads.

The Dallas FBI office was chosen for the investigation because he had already begun to infiltrating CISPES and the FBI wanted to centralize its probe, Varelli said.

The committee is considering filing a lawsuit against the government, Angela Sanbrano, CISPES national coordinator, said today in Washington at a news conference in front of FBI headquarters.

She said that FBI and Secret Service agents have contacted or visited numerous CISPES members and that ''we call on the FBI and other government ... agencies to end the harassment and illegal activities against us.''

Michael Ratner, a CISPES lawyer, said in an interview broadcast Wednesday by CBS News that ''the FBI has admitted to us that it has 17 volumes of information on the Dallas chapter of CISPES, and on the national chapter.'' Ratner said the committee has hundreds of other chapters around the country.

The committee has used a variety of tactics to attack the Reagan administration's policies of refusing to give political asylum to Salvadoran refugees, of training Salvadoran troops in anti-guerrilla tactics and in supporting a government in El Salvador that CISPES accuses of human rights violations.

Reagan administration officials say the committee is a front for the Marxist guerrillas fighting to overthrow El Salvador's U.S.-backed government.

Varelli said he also gave the Salvadoran national guard information about several political action organizations in the United States, including the Coalition for a New Foreign and Military Policy.

That group's director, David Reed, said he was frightened by the report.

''It scares me for a number of reasons,'' Reed told CBS. ''The national guard was a terrorist organization. What are they going to do with that information relative to the coalition?''

Varelli has sued six FBI agents and seeks to recover money he claims the agency owes him. He said he became disenchanted with the bureau partly because of the dispute.

At the meeting with CISPES members Wednesday, Varelli offered apologized to CISPES members for his activity.

''There was not one single criminal activity (I uncovered) in 3 years of investigations,'' he said.

''I was the first one who ever went into a CISPES group. I was convinced CISPES was an organization that needed to be stopped,'' Varelli told the group. ''But after I got to know you for 3 1/2 years, I felt differently.

''I don't feel you are communists or terrorists,'' he said.