City had thrown down the gauntlet to United as Edin Dzeko scored four times to destroy Tottenham at White Hart Lane on Sunday but Sir Alex Ferguson's team emphasised their determination to hold onto the Premier League crown with a Wayne Rooney hat-trick reducing beleaguered Arsenal to rubble in a sublime display at Old Trafford.
Ferguson once again fielded a youthful line-up and his gifted prodigies rose to the occasion in impressive style to record their third successive win of the season and move ahead of second placed City on goal difference at the top of the table.
"You feel humiliated when you concede eight goals. It was a terrible day for us," Arsenal boss Arsene Wenger said.
"It was a combination of an under-strength team and weakness. We collapsed physically in the second half."
Ferguson added: "It is (a surprise) because when you play Arsenal you expect a difficult game. If you look at Arsenal the team is weakened but we still did the job."
Danny Welbeck opened the scoring with a close-range header in the 22nd minute after Johan Djourou failed to clear Anderson's chipped pass and Arsenal squandered a chance to level when Robin van Persie's penalty, awarded for a Jonny Evans foul on Theo Walcott, was saved by David De Gea.
Younes Kaboul headed home for Spurs in the 68th minute, but the hosts -- who started with unsettled Chelsea target Luka Modric -- conceded a fifth when Dzeko curled into the top corner in stoppage time and Harry Redknapp's side sit bottom of the table after losing their opening two league games.
"We played a really good game against what is a fantastic team," Mancini said. "I was always sure Dzeko would score a lot of goals for us, he always scored in Wolfsburg."
"This is a very long season and it's important that we started well. The next games will be very hard because now everyone thinks we will score three or four goals, but that is impossible."
Elsewhere on Sunday, Newcastle striker Leon Best bagged a clinical double to fire his side to a 2-1 victory over Fulham at St James' Park.
Clint Dempsey got one back for Fulham late on but, three games into Martin Jol's reign, the west London outfit have only one point.
Stoke''s Ryan Shotton scored in the closing moments as the FA Cup finalists snatched a dramatic 1-0 victory against West Brom at the Hawthorns on Sunday.
Shotton netted in the 89th minute after a mix-up between goalkeeper Ben Foster and centre-back Gabriel Tamas to condemn Albion to their third successive Premier League defeat and give Tony Pulis''s undefeated side their first league win of the season.