It is claimed a man suffered a back injury when he was “violently flung” during a “peaceful protest” at Bramley Parish Hall, Rotherham.
The event, on Friday, was to inform the public about proposed changes to the HS2 rail line route in the area.
HS2 said the event was closed “for a short while” due to crowd numbers.
Richard Dickinson said his shirt was ripped when he was grabbed by a security guard.
“He grabbed hold of me, pushed me against the wall into a door, scraped me across the wall and just chucked me into the people who were coming inside the building,” Mr Dickinson said.
“It was totally out of order, we were protesting peacefully and this guy just suddenly lost his temper for no reason at all.”