Huge crowds, many without tickets, had congregated outside the stadium for the sell-out Premier League clash, West Ham's final home game before moving to the 60,000-seat Olympic Stadium after 112 years at Upton Park.
However, the carnival atmosphere was replaced by shameful scenes as some West Ham fans spotted United's coach on Green Street outside the ground and threw missiles, including a smoke canister, at the vehicle.
Several windows on the coach were damaged during the chaos and policemen with riot shields moved in to quell the trouble.
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Footage filmed by United's players on board the bus showed them cowering on the floor as they bus came under attack.
Police eventually managed to escort the United team to the entrance less than an hour before the scheduled 1845 GMT (1945 London time) kick-off and the match began at 1930 GMT.
"It wasn't nice, the coach got smashed up," said Manchester United captain Wayne Rooney. "I'm sure you will see the images but it was disappointing."
United manager Louis van Gaal looked visibly shaken by the incident as he walked into the stadium.
"The way we have been received is not the proper way of course," the Dutchman said. "That makes all the influence on the players and that is a pity."
West Ham's co-chairman David Sullivan, whose side ended an emotional night with a 3-2 victory, said United should have arrived earlier to avoid trouble.
"I think the police and officials have been kind, I would make them kick off at 7:45 if it was up to me. Manchester United know the score, they should have been here at four o'clock," he said.
FA PROBE
London's Metropolitan Police said in a statement that one officer and a member of the public sustained minor injuries, but no arrests had been made.
West Ham later issued a statement condemning those responsible for the disorder.
"We are aware that there were some supporters outside the Boleyn Ground who didn't act in an appropriate way when the Manchester United team bus was damaged," it said.
"That was not acceptable and we will work with the police to identify those responsible and ban them for life."
The Football Association also announced that it would launch a full investigation, including an incident where objects were thrown at United goalkeeper David de Gea during the game.
"The FA strongly condemns the unsavoury incidents this evening," it said in a statement.
West Ham have played at the 35,000-capacity Boleyn Ground since 1904 but will begin next season in the 700 million pounds Olympic Stadium after agreeing a 99-year lease.
(Editing by Tony Jimenez/Toby Davis/Peter Rutherford)
