The Monday Meadows Fire Club – an informal gathering of Edinburgh's fire performers, dancers and jugglers – met at the Pavillion Cafe last night but was interrupted after a gang approached members at around 10.30pm.
One rider asked what was happening before launching the bizarre attack on a member of the club.
An eyewitness said: “One of them came closer asking what is this and what's happening, when I answered him it's the Fire Club he threw ketchup all over my friend standing next to me then rode away.
"Two other guys were staying in the back and one of them was recording everything with a phone.
"Then one of our friends jumped on his bike to go home and the guys threw ketchup on him as well as he was leaving.
"Then we stepped forward, they tried to trash talk and tease us more, but after few minutes they left in direction of the tennis courts.
"They were all wearing balaclavas.”
The eyewitness to the latest Meadows incident added that he wished he’d filmed the incident.
He said: “I regret that I haven't thought of that at the time, but I was in shock. I would recommend that people record them as well so maybe it'll be easier to catch them based on their bikes and everything.”
Councillors for the Southside and Newington ward have been contacted for comment.