When Gary Hooper scored his second goal in the 49th minute — after Jambos midfielder David Obua had been sent off by referee Craig Thomson before the break for raising a hand to Charlie Mulgrew — a supporter came out of the main stand to grapple with the Parkhead boss.
An astonishing melee ensued and the intruder was eventually frogmarched up the tunnel by police but that was not the end of the trouble as some fans in the Celtic section of the ground appeared to fight with stewards and police.
There was more controversy to come when Kris Commons grabbed Celtic’s third in the 77th minute, moments after he was booked for simulation.
The Hoops midfielder was then sent off for leaving the field of play in celebration as the visitors finished the game one point behind Old Firm rivals Rangers going in to the last game of the season on Sunday.
Celtic’s win means the race for the Clydesdale Bank Premier League title will go down to the final game of the season, although the comfortable victory was overshadowed by the attack on Lennon.
Celtic assistant manager Johan Mjallby said that Lennon was “shaken” and that the attack could have been much worse.
“What has happened to Neil is a dark day for Scottish football and I’ve never seen anything like it, he is shaken but okay,” he said.
“I saw someone coming and we didn’t react at first, we were obviously shocked and the guy could have had anything in his hands.”
Lennon was approached by the fan after Gary Hooper had put the visitors two goals ahead.
Mjallby has now questioned how much more Lennon can put up with, even though he has a tough exterior.
“I haven’t really had time to speak to Neil, he has a strong character but how much can I guy take?”
“We hope he is going to be in a good mood later on tonight but I really dont know at this given moment. We have to look into it, this can’t happen again that is for sure, we all need to be secure in our jobs.”
Hearts also vowed to give their full cooperation to the authorities.
A club statement read: “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.
“We will give our full support to the stewards and police authorities in the ensuing investigation.”
The spotlight will in time turn to Motherwell’s visit to Parkhead and Rangers’ game against Kilmarnock at Rugby Park but for the next few days the Lennon incident will dominate.
Kello, Jonsson, Zaliukas, Bouzid, Craig Thomson, Obua, Black, McGowan, Skacel (Glen 77), Stevenson (Barr 77), Elliott (Driver 64).
Forster, Wilson (Cha 84), Loovens, Majstorovic, Izaguirre (Maloney 77), Ki, Brown, Mulgrew, Commons, Samaras (Murphy 85), Hooper.
Referee: Craig Thomson (Scotland).
Clydesdale Bank Premier League: Inverness CT 2 Hibernian 0, Motherwell 1 Kilmarnock 1.
Scottish League Championship First Division play-off semi-final, first leg: Brechin 2 Cowdenbeath 2, Forfar 1 Ayr 4.
Scottish League Championship Second Division play-off semi-final, first leg
Annan Ath 2 Alloa 1, Queens Park 1 Albion 1.
Barclays Premiership Reserve League play-off: Man Utd Res 1 Blackburn Res 2.