As well as Rooney's treble, Ashley Young helped himself to two goals, with further strikes coming from Nani and substitute Park Ji Sung after Danny Welbeck had scored the opening goal.
United's stunning display lifted the champions to the top of the Premier League with a 100 percent record from three matches, just hours after rivals Manchester City had issued their own statement of intent with a 5-1 win at Tottenham.
The embarrassing defeat illustrated the work required in the transfer market by Arsenal manager Arsene Wenger, whose position could be questioned after this demolition.
In an afternoon to forget for a makeshift Arsenal side, who saw Carl Jenkinson receive a late red card, Wenger endured one of his humiliating results of his long tenure with the Gunners.
Wenger's side have yet to win in the Premier League this season and this setback will send shockwaves through their camp.
While Ferguson's side celebrated scoring eight for the first time since 1999, it was Arsenal's worst ever defeat against United and the first time they have conceded eight in a league game since 1896.
Having seen Cesc Fabregas and Samir Nasri depart, and yet having found no replacements, Wenger was further hit by suspensions to Alex Song and Gervinho.
It meant he named a weakened side, with young French midfielder Francis Coquelin handed a baptism of fire on his Premier League debut.
United boss Sir Alex Ferguson kept faith with his own youngsters and two of them threatened to hand the hosts the lead inside the opening six minutes.
Tom Cleverley was narrowly wide off target with barely two minutes on the clock. Welbeck came even closer four minutes later as he beat the offside trap from Ashley Young's pass but failed to trouble Arsenal goalkeeper Wojciech Szczesny.
That was soon to change as Welbeck opened the scoring in the 22nd minute.
Anderson's cleverly weighted pass exposed the Arsenal defence as Welbeck shrugged off the attentions Johan Djourou before looping a header home.
Arsenal missed a glorious chance to respond five minutes later when Jonny Evans pulled down Theo Walcott for a penalty, but Robin van Persie's low spot-kick was turned aside by Manchester United goalkeeper David De Gea.
To add insult to injury, within 60 seconds Young had extended United's lead. as the winger struck from 25 yards.
The only blot on the copybook for Ferguson was a hamstring injury sustained by Welbeck, which will surely end his chances of featuring in England's forthcoming Euro 2012 qualifiers.
But even that couldn't halt the home side as Rooney's 41st minute free kick piled on the misery for Wenger.
Arsenal were given a glimmer of hope on the stroke of half-time when Tomas Rosicky's pass freed the England winger and he fired through De Gea's legs as the Gunners scored for the first time in the league this season.
Van Persie almost reduced the arrears further in the 53rd minute. Rosicky's pass freed the Dutch striker and his first-time volley was superbly kept out by De Gea.
That was as good as it got for the visitors, who conceded three goals in a seven minute burst.
Rooney curled home another 64th minute free-kick, Nani chipped home three minutes later before South Korean winger Park fired home.
Despite van Persie's strike two minutes later, there was worse to come for the shell-shocked visitors.
Jenkinson collected a second yellow card in the 76th minute after hauling down Javier Hernandez.
But Manchester United weren't done there as Rooney completed his treble from the penalty spot after Patrice Evra was fould by Walcott, before Young again found the target in injury-time to add to the visitors' torment.
Courtesy of Score Mobile FC (http://www.thescore.com)