CCTV captured the moment musclemen sprang into action after learning that the man had allegedly tried to swindle money from them by falsely claiming to be collecting funds for Bluebell Wood Children's Hospice.
They held him to the floor at the Muscle Madness Bodybuilding Gym in Sheffield until a team of police officers swooped to arrest him.
Now Helen Mower, head of fundraising for Bluebell Wood Children’s Hospice, has praised the gym members for their actions.
She said: "We’d like to say a big thank you to the gym members for suspecting this was not a genuine collection. We cannot praise their vigilance highly enough.
"Bluebell Wood doesn’t carry out door to door collections and we always ask anyone fundraising for us not to do these. We only do bucket collections at venues after we have got their permission in advance. We also give all our bucket collectors an official letter of authority."
The drama unfolded when a man dressed in a hospice t-shirt and carrying a bucket walked in to the gym at Effingham Road in Attercliffe at 10.30am on Monday and asked for donations.
But 51-year-old gym owner David Grubb said he became suspicious when the man couldn't provide proper identification and invited him back at 5pm when more gym members would be available to donate.
In the meantime he called police and the hospice who allegedly confirmed to him that a bogus caller was in the area.
When he returned later that day, bodybuilders made a citizens arrest until police arrived to take him away.
Mr Grubb, a father-of-five of Rotherham, said: "He chose the wrong gym. I'm not having people stealing from us."
Pictures of the incident have since gone viral on Facebook and have been seen by more than 85, 000 people.
The gym has since launched a fundraising page for Sheffield-based Bluebell Wood Children's Hospice - which cares for children with a shortened life expectancy in South Yorkshire - and it has so far raised more than £350 towards a £1000 target.
To donate visit https://www.justgiving.com/fundraising/Musclemadnessgymsheffield?utm_id=26