The Cup holders went down 4-1 to their Midlands rivals and later issued a statement to condemn the fan who charged on after Forest’s third goal and appeared to land punches before being bundled away.
“The club is appalled by such behaviour and offers its apologies to Nottingham Forest and its players whose safety was compromised,” Leicester said in a statement.
“We applaud the swift action taken by City Ground stewards in apprehending the individual involved, who will be subject to a lifetime ban from all Leicester City fixtures at home and away. We will offer every support to the relevant authorities in pursuing further appropriate action.”
Nottinghamshire Police later confirmed that a man had been arrested in connection with the incident.
“Investigations are in the very early stages and our enquiries into the circumstances surrounding the incident are ongoing,” a statement read.
It was the first meeting between the rivals for eight years and 4,000 Leicester fans made the short trip for the eagerly-awaited tie at a sold-out City Ground.
Asked about the incident, Leicester manager Brendan Rodgers said: “It’s behaviour we do not want to see. The field has to be a safe environment, we must always retain the respect in football and unfortunately the guy has come onto the pitch.
“The rules of society have changed — there’s no respect. People feel entitled to think they can do these things. We can’t have that lack of respect.”
Former Leicester striker and now BBC Sport presenter Gary Lineker described the fan as a “disgrace”.
“Whoever that so called fan was who ran on to the pitch throwing punches at @NFFC’s players should be banned for life by @LCFC. An embarrassment and a disgrace,” he said on Twitter.