Protestant victims' campaigner Willie Frazer was attacked close to a play park in Newry the day before he and relatives of the 10 men murdered at Kingsmill were due to stage a protest during a memorial event to Raymond McCreesh.
Stones and eggs were thrown at his car during the incident on Saturday.
The counter-demonstration at the park named after the IRA hunger striker was then cancelled.
Mr Frazer had planned to lead the protest at an event organised by dissident republican group Saoradh to mark the 37th anniversary of McCreesh's death in the Maze Prison.
But he called off the demonstration over fears for the safety of those attending.
McCreesh was caught in possession of a gun used in the 1976 massacre at Kingsmill.
Mr Frazer said: "Stones and eggs were thrown at my car on Saturday evening.
"We had wanted to make our disgust known at the glorification of an IRA killer on Newry, Mourne and Down District Council property.
"But there were real concerns for the safety of elderly people and young families, given the attack on myself on Saturday.
"I refused to put them in any sort of position where they might be threatened with violence, but this issue is not going to go away."
He said he will now contact the Equality Commission to question the reaction of the council to the staging of the commemoration, which he said had not received permission.
"As soon as the council were notified that Saoradh had planned this event they should have reacted but they refused to make any sort of comment. Instead it was only after repeated pressure that they issued any sort of statement at all," he added.
A statement from the council on Friday said: "Newry, Mourne and Down District Council has not received any request seeking permission to use council land for this upcoming gathering. Raymond McCreesh Park is a council play park."
However, Stephen Murney of Saoradh claimed it did not need the council's go-ahead.
"We have not asked permission from Newry, Mourne and Down District Council (or anyone else for that matter) because we are not obliged to," he told Newry.ie.
"Raymond McCreesh Park is a public park.
"It is owned by the people, it belongs to the public and the people are proud to have it named after Raymond McCreesh.
"Such gatherings do not need to seek permission.
"Over the years there have been several commemorations and vigils held in Ray McCreesh Park organised by various groups."
Last week Colin Worton, whose brother Kenneth died at Kingsmill, said it "beggared belief" that anyone could look up to McCreesh.
"It's more added insults - they are treating him as a hero, but he was anything but a hero," he said.
"How can you take a mass murderer as a hero?
"We even still have some republicans who don't believe the IRA carried out the Kingsmill massacre.
"It's been proven that they did, but many republicans still don't want to acknowledge that it was the IRA because it was such a heinous crime carried out in the name of republicanism."
Mr Worton accused republicans of double standards, referring to recent complaints from two Sinn Fein MPs about SAS flags flying in Loughgall on the anniversary of the 1987 ambush in which British special forces killed eight IRA men as they attempted to blow up the village's RUC station.
Meanwhile, on Twitter later on Saturday evening Mr Murney referred to the attack on Mr Frazer's car.
"Willie Frazer had a 'cracking time' in Newry tonight," he tweeted.
"At the time he was stoking sectarian tensions ahead of tomorrow's commemoration to honour Raymond McCreesh.
"Glad to see he was sent on his way."