Romelu Lukaku scored twice on his Premier League debut for United as the team threw down the gauntlet in an impressive opening day win.
Slaven Bilic's side were comprehensively outclassed as the Reds stormed to our biggest margin of victory for nearly three years with a ruthless display of pace, power and purpose.
Led by the immense performances of summer signings Lukaku and Nemanja Matic, the team brimmed with verve, creativity and flair throughout to fire a huge statement of intent to the rest of the league.
United started brightly and almost went ahead inside ten minutes with City loanee Joe Hart quickly tested on his Hammers debut.
He saved well from Juan Mata and then cleared under pressure from the Spaniard - after a wonderful pass from Paul Pogba - with Lukaku lurking.
The breakthrough was not long in coming, however, as the Reds deservedly went in front shortly after the half hour mark.
Matic won the ball high up the field and picked out Marcus Rashford whom in turn fed Lukaku with a perfectly weighted through ball.
The £75m man did the rest as he latched on to the pass and fired in an excellent finish off a post.
David de Gea had little to do but was called into action on 42 minutes when he palmed away Edmilson Fernandes' close range snap shot.
Rashford went close to a second shortly after the restart, but moments later United did move
2-0 ahead, with Lukaku again the main man.
Henrikh Mkhitaryan's inswinging set-piece picked out the Belgian, who this time did the damage with a header from eight yards.
Mkhitaryan had a goal disallowed for offside before Marko Arnautovic clipped the crossbar with a looping effort at the other end.
Not content with their comfortable two-goal advantage, United continued to swarm forwards and went close twice in quick succession.
Rashford thumped against the upright, Daley Blind volleyed narrowly over and Pogba lashed a shot wide from distance.
The reprieve proved only temporary for the overwhelmed Hammers as two late goals from a now rampant United side added deserved late gloss to the scoreline.
First, substitute Anthony Martial ran through and slotted beyond Hart having been played in by Mkhitaryan with three minutes to play.
Then, Pogba capped his sublime individual performance with a goal when he found the bottom corner with a low drive in the 90th minute.
We're Man United, we're top of the league!
Overall team performance: 9/10
United Faithful Man of the Match: Could easily give this to anyone in Red for a superb all-round team performance, but Nemanja Matic gets the nod (just) for an immense midfield display