TWICKENHAM, United Kingdom (AFP) - All Blacks centre Sonny Bill Williams gave his World Cup winners medal to a boy who he said was tackled by a Twickenham security guard after Saturday's final.

Williams said the boy was "smoked" - meaning thrown to the ground - by a security guard after running on to the field after the All Blacks beat Australia 34-17.

Williams said he felt sorry for the boy.

"Hopefully, (the medal) made it a night to remember for him," the All Blacks centre said.

"It's better that he has it rather than having it hang up at home."

The official Rugby World Cup Twitter account showed a picture of Williams giving his medal to "a young fan".

It did not say why however.

Williams came on as a half-time replacement for Conrad Smith and had an immediate impact.

He set up Ma'a Nonu for a try to push the All Blacks to a 21-3 lead.