Chicago office of the Federal Bureau of Investigation has nearly 7.7 million pages of files on suspected "subversive" and "extremist" groups and individuals - enough to make a stack more than twice as high as the World Trade Center or a string reaching from Pittsburg to Denver.

The extent of the files, which are maintained on the ninth floor of the Everett M. Dirksen Federal Building here, was revealed in affidavits by two FBI officials made public yesterday in connection with three U.S. District Court civil suits that charged the FBI with illegally spying on persons and groups engaged in lawful political activities.

FBI documents previously made public in connection with the suits reveal thses groups are among those on whom files are kept under the "subversive classification: the American Civil Liberties Union, the Chicago Committee to Defend the Bill of Rights and the National Association of Social Workers.

Among those on whom files are kept under the "extremist" classification are the Rev. Jesse L. Jackson's operation PUSH, the Afro-American Patrolmen's League, the National Association for the Advancement of Colored People and the Congress on Racial Equality.

The affidavits revealing the extent of the files were made by Thomas E. Vornberger and James W. Awe, top officials of the FBI's records management branch in Washington.

Vornberger's affidavit says the FBI's Chicago office - one of 59 field offices nationwide - has under the two classifications 3,207 linear feet of files, including 802 feet opened after Jan. 1, 1966.

The total is enough to make two stacks as tall as the 1,350-foot World Trade Center and a third stack that would reach slightly beyond the 40th floor of the 110-story twin-tower complex.

Awe's affidavit says there are about 200 pages per inch in the average file. This means there are almost 7.7 million pages, including 1.9 million opened since Jan. 1, 1966.

Laid out to end that would be enough 8 1/2-by-11 inch sheets to stretch 1.336 miles, or slightly more than the flight distance from Pittsburg to Denver.

The extent of similar files in the FBI's 58 other field offices is unknown.

The costs of gathering the information in the Chicago files were staggering. Just to photocopy the pages would require more than a million man-hours and cost more than $8 million, according to Vornberger's affidavit.

It has been disclosed that from 1966 to 1976, the Chicago FBI office paid out more than $2.5 million to 5,145 "informants" and "confidential sources" who contributed information to the files.

During that 10-year period, the FBI disclosed, the Chicago office opened files on nearly 27,900 organizations and individuals on whom there had been no files.

The FBI said it checked a sample of to 1976 and found that 84.6 percent were on individuals and 15.4 percent were on organizations.

There are four times as many subversive files as extremist files, the FBI estimated. Those classifications do not include any sedition, sabotage or other criminal investigative files.