Weyman Bennett of Unite Against Fascism was among those leading the protest.
He said: “As soon as the BNP showed their face here what did we do? We smashed them. We have to speak for the six million murdered in the Holocaust. We are the majority, not the BNP.
“When Hitler rose people did not know who he was. That building over there [Parliament] is not for Nazis or racists.”
Mr Bennett fired a warning to the BNP, saying: “Wherever you go in this country you will be welcomed by protestors like this. We are sending you a message: You will not follow in the steps of Hitler, Le Pen and Mussolini.”
One protestor was later taken away in an ambulance.