The Blades captain was sidelined for the second leg because of injury and was standing on the touchline after Forest secured a dramatic 3-2 penalty shootout victory following a 3-3 aggregate draw, which precipitated a pitch invasion at the City Ground.

Footage shared on social media appears to show one supporter running at the 36-year-old former Forest striker and knocking him to the ground.

"A 31-year-old man has been arrested on suspicion of assault in connection with this incident. He remains in custody and will be questioned by officers," Nottinghamshire Police's chief inspector Paul Hennessy said.

"Our enquiries into the circumstances surrounding the incident are ongoing. We have spoken to the player concerned and we are working with both clubs who are supporting us with our investigation."

Sharp required stitches after the incident which a furious Sheffield United boss Paul Heckingbottom labelled "an assault."

"He's shook up, bleeding, angry," Heckingbottom said.

"Bill was minding his own business off the pitch, trying to get our players away. He was blindsided, it was cowardly. He was knocked to the ground (and required) stitches, he was shook up."

Forest have condemned the incident and say the offender will be subject to "a life ban" from the club.

"Nottingham Forest Football Club are appalled to learn that our former player, Billy Sharp, was assaulted leaving the pitch after tonight's match at the City Ground," the statement added.

"The club would also like to apologise to Billy personally and to Sheffield United Football Club."

The Blades had dragged themselves back into the tie after losing the first leg 2-1 and trailing on the night to Brennan Johnson's opener.

Second-half goals from Morgan Gibbs-White and John Fleck took it to extra time and after another 30 minutes and still 3-3 on aggregate, Congolese goalkeeper Brice Samba denied Oliver Norwood, Conor Hourihane and Gibbs-White in the shootout to send Forest through to a showdown against Huddersfield at Wembley Stadium on Sunday May 29.

Forest, bottom of the second tier after seven games when Steve Cooper took charge in September, are now just one win away from a return to the top-flight after a 23-year absence.

"I would have loved to have won the game 3-1 and walked off but the fact we've done it like we have means we're more tooled up now to go to the final," Cooper said.

"I promise you I will work even harder than I've worked to get this. No days off, I'll be the first one at the training ground (on Wednesday) and we'll be ready.

Australian Associated Press