Al Davis, the only National Football League owner who is not a defendant in the trial, testified today that the NFL engaged in "illegal" acts to thwart the U.S. Football League.

These acts, testified the Los Angeles Raiders' managing general partner, sent the NFL "heading down a very dangerous path . . . headed into this courtroom."

In what several USFL officials viewed as some of the most damaging testimony against the NFL in this seven-week-old antitrust trial, Davis said that the NFL was part of an "illegal collaboration" with the city of Oakland and the Oakland-Alameda County Coliseum to "destroy" the now-dormant Oakland Invaders of the USFL.

Davis, who moved his Raiders from Oakland to Los Angeles in 1982, testified that Oakland city officials had a greater interest in garnering an NFL franchise than in helping the Invaders. He said that, after the Invaders' first home game ever, in 1983, an overtime loss attended by 41,000, Oakland Mayor Lionel Wilson was quoted as saying the Invaders played a "dull" game.

Furthermore, Davis testified that when the Invaders opened their 1985 home schedule, Wilson and other city officials did not attend the game. Rather, they held a cocktail party at an NFL owners meeting in Phoenix to maintain their interest in acquiring an NFL franchise. Davis also said that the Invaders' lease stipulated that, if the city acquired an NFL team, that team would be given preference in scheduling.

USFL officials felt Davis' testimony was all the more important in that it provided the six jurors, none of whom claimed to have a great deal of knowledge about the professional football industry, with an NFL owner making charges against his own league and, in so doing, supporting charges made earlier by USFL officials.

One NFL owner said outside the courtroom, however, that he felt relieved that Davis hadn't dropped any "bombshells" today. Davis was limited in the scope of his testimony today by Judge Peter Leisure and was not allowed to testify about the Raiders' litigation against the NFL.

Davis also testified that "there was an understanding among NFL owners that there would not be a third team in New York" besides the Jets and Giants, both of whom play in New Jersey. USFL attorneys hoped to use this testimony to prove their so-called "New York conspiracy," in which, they allege, the NFL misled New York government officials into believing that an NFL franchise would soon move into New York City.

Davis' testimony on this subject prompted a heated conference between opposing lawyers and Leisure after the session adjourned and the jury had left.

NFL lawyers argued that USFL lawyer Harvey Myerson has failed to prove the "New York conspiracy," which, in his opening statement, he had said he would accomplish. Myerson disputed this contention. It's possible that the NFL attorneys will request through a legal brief that all references to the "New York conspiracy" be stricken from the record.

Little of Davis' testimony dealt directly with the relationship between the television networks and the two leagues, which Leisure termed the central issue of the case in a pretrial order. The USFL has filed a $1.5 billion suit alleging the NFL illegally "tied up" all three networks. Since abandoning its spring schedule and planning a fall schedule, the USFL has been unable to get a network contract and, along with damages, is seeking to force the NFL off at least one network.

Davis spent more than two hours on the witness stand today, including just 10 minutes of cross-examination by attorney Frank Rothman of the NFL. Rothman's cross-examination was brief because he was limited to questioning Davis only on today's direct testimony.

It's possible the NFL will call Davis as a witness, too. He said outside the courtroom that he "absolutely" would return, if requested. Sportscaster Howard Cosell will be the USFL's last witness, on Wednesday. The NFL is expected to call former USFL commissioner Chet Simmons as its first witness Thursday. The trial is expected to end about Aug. 1.

Davis testified that he wanted the Invaders to succeed in Oakland. He said that former Raiders players and coaches joined the Invaders, as did the former Raiders ticket manager. Davis said that the Invaders even inherited 10 of the Raiders' Bay Area booster groups and struck a deal with the Raiders' former radio station.

"We tried in every which way to get them Invaders started," he testified. "We thought they had a very viable commodity and a very viable league."

Correspondent John Kennedy contributed to this report.