Aaron Smith from Hemel Hempstead, who was at the game, said: "I saw one man cornered by at least a dozen Millwall fans before kick-off. Terrifying scenes."
Eyewitness Fergal O'Brien said: "I saw a West Ham fan getting punched in the face by a burly bald man who had a coin between his fists."
Another fan said: "I was in a throng walking up and a group of hooligans came for us.
"The police wanted to help but they were clearly afraid to use force. I am still shaking. I thought I was going to die."
Ben Newton, from Harrow, north London, said his father had been stretchered away after being hit with a dart to the head.
And a Millwall fan told the BBC: "It was like a war zone outside the stadium. There was fighting all round.
"I brought my kids with me tonight and they've seen some violence that is indescribable."