The Washington Bullets, who had lost to some of the worst teams in the National Basketball Association, ended their nine-game losing streak in grand style last night, beating the Boston Celtics, 93-91, in a Capital Centre cliffhanger.

Center Rick Mahorn, a 53 percent free throw shooter, sank two free tosses with seven seconds remaining for the winning points.

"I knew he was going to make them," said teammate Carlos Terry, who also was instrumental in the victory. "I could see in his eyes that he wanted it."

Earlier in the day the Bullets signed free agent guard Ricky Sobers and released Chubby Cox. Sobers was in uniform, but didn't play. He wasn't needed.

The Bullets, surprisingly, came out running and moved to a 13-point lead in the second quarter before the Celtics settled down and began chipping away at the lead behind the scoring of Larry Bird and Kevin McHale.

Boston went into the fourth quarter with a 75-74 lead and built it to eight points on a layup by Gerald Henderson with 4:15 remaining.

The Bullets, who scrapped and clawed all game long, responded with a 10-0 burst, holding the Celtics scoreless until there were only 11 seconds to play.

Frank Johnson started the flurry with a 10-footer and, after an 18-footer by Terry and a layup by Jeff Ruland, the Celtics called for a timeout with 2:17 left and their lead down to 89-87.

The timeout didn't help, as Robert Parish missed a turnaround jumper and Terry came down with the rebound. Johnson made a base-line jumper to tie the score at 89 with 69 seconds left.

McHale, who started the game in place of Cedric Maxwell, out with flu, then had a pass batted away by Ruland as he was trapped on the basline. The ever-present Terry was on the spot to pick up the ball.

At the other end, the Bullets passed inside to Ruland and he was fouled by McHale with 28 seconds left. He made both free throws for a 91-89 lead.

Parish tied the game at 91 with two free throws after he was fouled by Ruland with 11 seconds to go.

The Bullets called time to get the ball at midcourt. Terry in-bounded it to Ruland, but Bird knocked Ruland's pass, intended for Greg Ballard, out of bounds with seven seconds to play.

Ballard in-bounded the ball from under the basket and McHale reached over Mahorn's back, fouling him.

"I just went up there and said I was going to make them," said Mahorn. "The only thing I did differently than I usually do is I took a deep breath, that's all."

The Bullets, the worst free throw shooting team in the NBA at 68.3 percent, made nine of 10 foul shots in the fourth quarter.

After Mahorn made his free throws, the Celtics called time out to plan their last shot. But Scott Wed-man's three-point try from the left corner was short, Ruland rebounded the ball and dribbled away as time expired.

Spencer Haywood was high for the Bullets with 19 points and 10 rebounds. Ruland had 16 points and 14 rebounds, Ballard 18 points.

Bird had 22 points and 16 rebounds for Boston, but the Bullets won the battle of the boards, 55-44.

"We really needed a boost in the confidence department, and beating one of the best teams in the league should really give us one," said Coach Gene Shue, who exchanged high fives at midcourt with his assistants, Bernie Bickerstaff and Don Moran, after the game.

The Celtics have the third-best record in the NBA and had won 12 of their last 13.

Terms of Sobers' contract weren't announced, but it is believed to be a three-year deal paying the seven-year veteran, who has a 14-point career average, $300,000 a year. That would make him one of the highest-paid players on the team. Sobers played the last three seasons with the Chicago Bulls. The Bullets gave the Bulls second-round draft choices in 1983 and 1985 as compensation.

Washington still has two first-round and two second-round draft choices in 1983.

"We picked up Ricky to stabilize our back court, where we have had serious injury problems all season long," said General Manager Bob Ferry. "He's a very good defensive player and a very good open shooter. He has the ability to play both the big- and small-guard positions."

Sobers was drafted on the first round by Phoenix in 1975 after playing collegiately at Nevada-Las Vegas.

His 85 percent free throw shooting ranks him sixth among active players and 10th all time.