One witness told the Daily Mail: "Ten to 15 people dressed all in black, with black hoods and black face masks, leapt over the barriers and instantly engaged in a fight with two or three security guards.
"They tried to stop them but they just started punching them in the face.”
The Ayn Rand Institute, named for the author of The Fountainhead and Atlas Shrugged, advocates objectivism, a philosophical system centred on “rational self-interest” and laissez-faire capitalism.
Mr Benjamin has been criticised for his “anti-progressive” politics, and his online targeting of feminists and Black Lives Matter activists.
He was at the centre of a 2016 media storm after he was accused of instigating a misogynistic trolling campaign against Labour MP Jess Phillips.