An Environmental Protection Agency official testified yesterday that Washington taxicab driver Jack B. Dembo chased him, knocked him down and pinned him to the sidewalk during a dispute over the amount of a cab fare.

David R. Berg, 34, the manager of an EPA energy research program, told a Washington Metropolitan Area Transit Commission hearing the episode occurred last Dec. 8 after Dembo tried to charge him $7.80 for a trip from National Airport to the agency's office at 401 M St. SW that Berg maintained should have cost about $4.50.

When police officers arrived in response to a bystander's call, Berg said, they refused to listen to his complaint and insisted that he pay Dembo. Berg said he threw down his briefcase in frustration, only to be handcuffed, taken to the police station and charged with disorderly conduct. He said the charge was later dropped.

Berg's testimony was corroborated by Wayne A. Holroyd, then a cook at a Southwest Washington restaurant, who said he called the police.

Dembo, testifying later, acknowledged that he chased and stopped Berg, fearing the passenger was going to get away without paying. "Mr. Berg was calling me all kinds of names," said Dembo, 41, who contended the actual fare requested was $5.25 and Berg wanted to pay only $3.

The conflicting testimony was offered at the last of three days of transit commission hearings conducted by Administrative Law Judge Francis Welch into a formal accusation that Dembo overcharged passengers on at least 11 interstate trips in the past two years, most involving travel to or from National Airport in Virginia.

The transit commission sets fares for trips across state lines in the region. Since it has no enforcement power, it filed a formal case against Dembo to decide whether it should recommend that the D.C. government suspend or revoke his hacker's permit and possibly prosecute him for criminal violations. Judge Welch said yesterday the commission will decide sometime after June 19, when final briefs are due.

In dollar terms, the most serious accusation against Dembo was made in testimony April 21 by Peggy Ann Potts of Bowie. She said Dembo charged her and a professional associate $74.20 for a 1979 trip from the airport to two destinations in Bowie. The transit commission staff said the trips should cost about $21. As in most other disputed cases, Dembo refunded the alleged overcharges.

Dembo, who then drove for Yellow Cab of Washington and now drives for Dial Cab, admitted when questioned yesterday that he gave receipts to Potts which falsely identfied him as a driver for Yellow Cab of Alexandria.

Most of the disputes involving Dembo entailed questions about whether the passengers were traveling as a group, which results in discount fares, or as individuals. Dembo testified yesterday that he nearly always assumes passengers are traveling individually unless they tell him otherwise, and that this was the case in the Potts trip.