The Manchester United team bus has been pelted with objects as it entered Upton Park in London.
Video footage showed pieces falling off the side of the coach as objects were thrown and a crowd stood filming on their phones.
The Boleyn Ground, which has been home to West Ham United for 112 years, is hosting its last match on Tuesday evening.
Kick-off has been delayed until 8.30pm and Metropolitan Police officers are at the scene.
United captain Wayne Rooney told Sky Sports: “It wasn’t nice. The coach got smashed up, but we are here now. I’m sure you’ll see the images.
“It’s not for me to say, but it was disappointing of course. It’s a big night for West Ham, but I’m sure West Ham as a club will be disappointed with what the fans have done.”
West Ham co-owner David Sullivan said he was disappointed that fans would be delayed.
He told Sky Sports: “I’m gravely disappointed, more for our supporters. What people forget is they’ve got trains and buses to get home and we’ve got a wonderful, wonderful show after the game for them. Many just can’t stay any more.
“Man United should have got here at 4pm. They know it’s our last game. They had the same problem at Tottenham a few weeks ago. I think the police and officials have been kind. I’d make them kick off at 7.45pm if I was in charge.”