The second goal came after Gallas needlessly fouled French countryman Patrice Evra a few meters outside the Arsenal area and Hargreaves curled the free kick inside the near post with Lehmann making no move.
Asked why United's other free kick specialist, Ronaldo, didn't take it, Hargreaves said the distance was more in his range.
"Ronny took the first one," Hargreaves said about a first-half Ronaldo free kick which was blocked by the Arsenal wall. "He said 'it's too far for you.' When the second came it was a bit closer and a good distance for me so I didn't ask him."
In Sunday's other game, Fernando Torres scored his 30th goal of the season and Liverpool beat Blackburn 3-1 on to strengthen its hold on fourth place in the Premier League.
Steven Gerrard and Andriy Voronin also hit the target for the Reds at Anfield where Roque Santa Cruz replied with an injury-time consolation strike for Rovers.
The victory gives Liverpool 66 points from 34 games and a five-point advantage over neighbor Everton in the race for the fourth spot, which means a shot at next season's Champions League.
Liverpool's form on the field appeared unaffected by the escalating conflict between co-owners Tom Hicks and George Gillett Jr. over the future of chief executive Rick Parry, and Rafa Benitez' team is also looking forward to a Champions League semifinal showdown with Chelsea.
Liverpool appeared low key in the early stages, however, and Blackburn striker Jason Roberts had the ball in the Liverpool net in the sixth minute, a goal TV replays showed was wrongly ruled out for offside.
Liverpool took control after Gerrard's goal in the 60th minute.
The captain swapped passes with Torres on the edge of the Blackburn area and ran past two defenders to fire past goalkeeper Brad Friedel.
Torres made it 2-0 in the 82nd, heading home a rightwing cross from Gerrard for his 22nd league goal of the season and Voronin turned home a low cross from the right by John Arne Riise.
Santa Cruz produced a spectacular volley in the 93rd for Rovers, which stayed ninth and squandered a chance to overtake Manchester City.
Saturday's results were: Birmingham 1, Everton 1; Bolton 1, West Ham 0; Derby 0, Aston Villa 6; Reading 0, Fulham 2; Sunderland 1, Manchester City 2; Tottenham 1, Middlesbrough 1; Portsmouth 0, Newcastle 0.
(Agencies April 14, 2008)