United dominated the Gunners who suffered their worst ever Premier League defeat.
With Manchester City winning 5-1 at Tottenham also on Sunday, it proved a superb day for the two Manchester clubs, who are level at the top with a maximum nine points from their three games.
In contrast, north London rivals Tottenham and Arsenal are struggling at the other end of the table, with Tottenham at the bottom without a point and Arsenal fourth from bottom with just one point.
Arsenal, who sold star players Cesc Fabregas and Samir Nasri this month, were always struggling to cope with the home side once Danny Welbeck headed United into the lead following hesitancy in the defense.
United soon made them pay when Ashley Young curled home a superb second before Rooney’s brilliant free-kick made it 3-0.
Walcott pulled a goal back for Arsenal just before half-time from an acute angle, but United restored their advantage with another trademark Rooney free-kick in the 64th minute.
By now, United were running riot, and Nani made it 5-1 after being left free in the penalty area to clip the ball into the net.
Substitute Park Ji-Sung added a sixth before Van Persie scored from close range to make it 6-2.
Arsenal then had defender Carl Jenkinson sent off for a second yellow card — meaning the club have had a player dismissed in each of their first three matches. And soon after, Rooney completed his hat-trick from the penalty spot after Walcott’s foul on Nani.
The humiliation was completed when Young matched his first-half strike with another superb effort in injury time.
The defeat, the first time Arsenal had conceded eight goals in a league game since 1896, increases the pressure on manager Arsene Wenger, with new signings expected to arrive before the deadline for trades on Wednesday.
“Of course it hurts, it’s humiliating, but you could see that we had not recovered physically in the second half from Wednesday night’s win over Udinese,” Wenger told the official Arsenal website.
“We were short in some areas, that is for sure. They have class and they punished us. It was 3-1 at half-time and I think that was harsh against us.
“We missed a penalty, we had a chance to get back to 3-2. We tried desperately to get back but we opened ourselves up and were punished. Their finishing was great today,” added the Frenchman.
United’s performance was matched by the dazzling display of big-spending rivals Manchester City, with Bosnian striker Edin Dzeko scoring four goals in their rout of Tottenham at White Hart Lane.
With Frenchman Nasri superb on his debut following his move from Arsenal, City were too strong for Spurs, and they went ahead in the 34th minute when Dzeko scored from Nasri’s cross.
And the duo combined again seven minutes later when Dzeko scored with a clever back-header from Nasri’s cross.
Dzeko’s completed his hat-trick early in the second half when he tapped in Yaya Toure’s cutback.
The home side pulled a goal back when Younes Kaboul headed in Rafael van der Vaart‘s corner.
But Sergio Aguero soon restored the three-goal advantage with a brilliant drive from outside the penalty area.
And Dzeko curled home his fourth and Manchester City’s fifth in injury time.
The result leaves Harry Redknapp’s side without a point along with Blackburn and West Bromwich Albion, who suffered their third straight defeat 1-0 at home to Stoke Sunday.
In Sunday’s other Premier League match, Newcastle beat Fulham 2-1 at home with two goals from striker Leon Best