Scottish Football Association chief executive Stewart Regan has lambasted the fan who attacked Celtic manager Neil Lennon on Wednesday.During Celtic's 3-0 win at Hearts that kept their slim Scottish Premier League title chances alive, a fan ran onto the pitch and made attempts to grab Lennon, who kicked the fan in response.

Celtic assistant manager Alan Thompson was quickly on the scene and intervened before the man was wrestled to the ground by security staff and police and taken up the tunnel. He was later arrested.

The incident enraged some Celtic fans, who began fighting with stewards and police. Reports also suggest pro-IRA songs were sung loudly by the travelling support, despite rivals Rangers being recently punished by UEFA for sectarian chanting.

''Clearly this kind of behaviour from supporters is wholly unacceptable," Regan said.

''The safety of players, club officials and match officials is paramount on or around the field of play, and this clear breach of security is a matter I will be discussing with SPL chief executive Neil Doncaster tomorrow.''

Hearts are likely to face punishment for not having adequate security measures in place to stop the attack on Lennon, with the club immediately announcing that an investigation had already begun.

"An investigation into events which took place in the second half of tonight's game against Celtic is now underway," a club statement said.

"Hearts is a club which prides itself on its 'football for all' policy and these sorts of actions have no place in the game of football."

The police will also be sure to investigate the issue, with Lennon having been under close watch from the authorities throughout 2011.

He has already had bullets and parcel bombs sent to him through the post, while suspicious packages have also arrived at his place of residence.

"A number of incidents took place during this match including some violent disorder. Lothian and Borders Police will fully investigate all of the incidents that occurred at the match this evening and will work closely with both football clubs and the SPL," a police spokesman said.

Lennon, a known Roman Catholic, retired from international football in 2002 after he voiced his desire to play for a team representing a United Ireland.

His stance led to death threats from fans of the country he did represent, Northern Ireland.