A busy Capitol Hill horse race and numbers betting operation has flourished in a Longworth House Office Building corridor just outside its employee cafeteria, according to federal law enforcement officials who watched the gambling operation for weeks.

Acting on a tip from two young free-lance reporters - one of whom made bets himself in trying to expose the operation - investigators from the FBI and the U.S. Attorney's Office here observed the gambling activities from late February until two weeks ago.

They were getting ready to arrest an elderly bookie who collected the bets, FBI agent Elmer Todd said yesterday, when somebody tipped the bookie and the operation was suddenly shut down.

The federal investigation of gambling on Capitol Hill is continuing, however, according to both prosecutors and the FBI. The U.S. Attorney's Office is now reviewing the evidence already gathered to determine what legal steps to take.

FBI agent Todd said those placing bets with the Longworth building gambling operation were "from various walks of life, high and low." However, he and the two free-lance reporters said no members of Congress were seen placing bets.

At one stage of the investigation, according to Todd, investigators stationed themselves in congressional office building telephone booths trying to "catch the number" of where the bookie phoned his bets to each day.

Todd said he could not provide information on just how much money was involved each day in the gambling activity. "But, before a race," he said the bookie would "go to phone in the bets and sometimes he'd be in that booth as long as 45 minutes. He was a pretty active fellow."

The two free-lance reporters, Lewis Perdue and Ken Cummins, who work for the recently organized Congressional News Syndicate - one of a growing number of small, independent news gathering groups on Capitol Hill - moved to make their undercover reporting scoop public last week.

They sent a story about the gambling investigation and their role in it to several newspapers, including The Washington Post. Details then emerged in interviews with federal and Capitol law enforcement officials.

Perdue and Cummins said they watched as the bookie collected betting slips and money from Capitol Hill employees, including congressional aides, committee staff members and Capitol Police officers. One of the reporters, they said, made bets of his own.

The bookie, according to Cummins, told the undercover reporter that the possible pay-off was 600-to 1 on a $1 to $3 bet and that winners could collect their money "in the Folding Room" - a reference to the House Publications Distribution Room, which is around the corner from the cafeteria in the basement of the Longworth building.

During a six-week period beginning Feb. 4, the undercover reporter placed a total of seven bets on either numbers games or horses running at Bowie Racetrack. He once "tried to place a bet on an NCAA basketball game" but was told by the bookie that he didn't take bets on basketball games.

"We kept hoping we would win so we could check out pay-off operations in the Folding Room," said Cummins, but they never won.

Eli S. Bjellos and Frank Bechte, the director and deputy director, respectively, of the Publications Distribution Room, both denied to The Post that their operation was used as a gambling pay-off location.

"We wouldn't allow anything like that to go on," Bjellos said. "We know our people. I go through the place everyday."

Two employees of the Folding Room said through a Capitol Hill employee, however, that they knew about the gambling operations in the Longworth building and that the bookie set-up near the cafeteria was common knowledge among many employees. But they declined to discuss details with a reporter.

Both Bjellos and Bechtel acknowledged knowing the man identified by federal investigators as a bookie. They said they had seen him around the hallways of the office building for years, but did not know what he did for a living.

FBI agent Todd said yesterday that investigators kept the cafeteria area and the bookie under surveillance for several weeks and were "trying to locate the overall gambling organization setup" before making any arrests under federal interstate gambling statutes.

Just when federal agents were preparing to obtain a warrant to arrest the man, Todd said, "somebody tipped him" and the gambling activities stopped. Federal agents were still able to determine whom the bookie was turning his money over to, Todd said, and the FBI is keeping the case open.

Todd said he could not personally verify the two free-lance reporters' accounts of Capitol Hill police officers placing bets. But he described the bookie as "a fixture around there. He was certainly known to the police and they were often seen speaking to him."

Perdue and Cummins said that FBI agents notified House Sergeant of Arms Kenneth R. Harding who, in turn, notified House Speaker Thomas P. (Tip) O'Neill (D-Mass.) about the investigation.

Harding told The Post he would not say what proceduures he followed after being informed of the investigation. But he noted, "there's very little that goes on on the Hill that the leadership doesn't know about."

O'Neill's administrative assistant, Gary Hymel, said information about the gambling investigation "stopped with me. I didn't tell the speaker, so the speaker didn't know."

James M. Powell, chief of the U.S. Capitol Police, said that, during the 12 years he has held that post, there have been other complaints about gambling on the Hill and he brought in undercover D.C. police officers to investigate them. No arrests have ever been made, however, he said.

Powell said he did not have any proof of allegations that his officers are permitting or participating in the gambling activity "but if this has been going on, it's apparent we're missing something."