In one of the most remarkable reversals of form in NBA history, the Philadelphia 76ers emphatically ended Boston's hopes of repeating as champion today with a brilliant all-around performance.

After stumbling through one of their worst games in recent years Friday night, the 76ers came back and overwhelmed the Celtics in every phase of the game, winning, 120-106, in the seventh game of the Eastern Conference finals and advancing to the championship series against Los Angeles.

That best-of-seven series starts Thursday night in Philadelphia (WDVM-TV-9 at 9). The Lakers and 76ers met in the championship series in 1980, with Los Angeles winning, 4-2.

Had they been defeated today, the 76ers would have lost the conference finals to the Celtics two years in a row after leading the series, three games to one. They lost Game 5 by 29 points and scored only 75 points when they were defeated in Game 6 Friday night.

Coach Billy Cunningham, obviously upset by the stories in the newspapers here and in Philadelphia after Game 6, including a Sunday morning headline that took up most of the front page of a tabloid paper saying: "Will Sixers Choke?", had little to say to the media afterward.

"I'm ecstatic for the 12 guys in there," he said, referring to his players in a nearby dressing room. "They stuck together when everybody else had buried us. Period. Goodbye."

Andrew Toney scored 34 points for the Sixers and Julius Erving, averaging only 17 points per game in the series, had 29. Robert Parish scored 23 to lead the Celtics.

Despite the enthusiastic encouragement of a sellout crowd of 15,320, the Celtics never showed the intensity, the quickness or the aggressiveness they displayed in winning the previous two games. They were tentative and passed as if they had never seen each other before. They had 22 turnovers.

"We've been in a lot of big games and I don't believe that we passed the ball as fundamentally poorly as we did today," Coach Bill Fitch said. "Our passing was just atrocious."

The 76ers took advantage of almost every mistake, turning them into 29 points.

"Our defense carried us," said forward Bobby Jones. "And Andrew (Toney) did the rest."

Toney, a 6-foot-3 guard who had made only one of 11 shots in Friday night's humiliating 88-75 defeat, scored 14 points in the first quarter and finished only five points under his playoff high.

"I was determined to have a good game after Friday night," Toney said. "A couple of guys called me at home and offered some encouragement. That meant a lot and I sure didn't want to let them down."

Several 76ers said it was very important not to let the Celtics get off to a big lead, because if they did it would inspire the fans. Although Larry Bird (20 points) made Boston's first four points and the Celtics led, 7-4, Philadelphia rallied and built a 21-14 advantage after 7 1/2 minutes.

"A lot of people counted us out, but we knew we could win," said Maurice Cheeks, who had 19 points and 11 assists. "Sure, there was some doubt after Game 6, but we've come back before. When we rebound, block out, do everything we have to to win, this is what happens."

Erving, who scored 20 points in the second half today, called a meeting Saturday afternoon after a workout in the Palestra and stressed several things that he thought contributed to the 76ers' stunning performance.

"I think it had a jelling influence," Erving said. "We ironed out some things we were doing offensively and encouraged a 12-man effort. We tried to put Friday's game out of our minds."

Erving, an all-star forward, started very slowly. He was scoreless when taken out with 44 seconds left in the first quarter and didn't make his first shot until 7:12 remained in the half.

Once he started making his outside shots, it opened up the court and gave the 76ers the offensive balance they had been lacking. He had nine points in a 3:15 stretch when his team took a 46-40 lead. After falling behind, 50-42, Boston scored seven of the last nine points of the period to reduce its deficit to 52-49 at intermission.

"All year long we've talked about short-range goals," Erving said. "So at halftime, we said that if we could just play the game as well for the next 24 as we did in the first half, we'd win. Then we wound up playing better."

Philadelphia scored the first six points of the second half on two shots by Cheeks and a fast-break dunk by Erving that forced Boston to use a timeout. The Celtics came back with four quick points, but the 76ers refused to fold.

Erving scored four more points, Cheeks broke away for a layup and the 76ers had their first double-figure lead, 64-54, with 7:43 left in the quarter. Once again, the Celtics made a move, scoring eight straight points, but again the 76ers came back, with six straight.

Erving made a base line jumper and an 18-footer with a second remaining and Philadelphia took an 83-71 lead into the fourth quarter. Nevertheless, the crowd, obviously remembering the 76ers' history, was on its feet cheering.

A three-point play by Toney, a fast-break dunk by Darryl Dawkins, a 14-footer by Toney and a jumper by Erving increased the 76ers' lead to 96-80 when time was called with 7:19 to play.

"That's when I felt we had the game," Erving said. "I knew they'd make another run at us, but I figured if we scored just 10 more points, they'd have to score 26 and they didn't have time to do that."

So, time ran out on the Celtics, just as it has for the last 12 champions, who also failed to make a successful defense.