Police had to intervene as a scuffle broke out moments after Labour shadow chancellor John McDonnell’s re-election was confirmed.
Footage from the Hayes and Harlington count, in Uxbridge, West London, showed angry exchanges between a handful of people.
Cries of “liar” and “terrorist” rang out as Mr McDonnell was giving his acceptance speech.
Police had to intervene to deal with the disturbance.
In his victory speech, the Labour veteran said: “It is quite clear that there are fears also now for our democracy and the rise of the extreme right.
“But I say now we will organise and we will ensure this type of democracy will reign, and the democratic wishes of our people will be respected, and this sort of extreme right wing violence will not be tolerated within our society.”
Several people with lanyards on had to be pulled apart, with bystanders capturing the incident on mobile phones.
Speaking afterwards, Mr McDonnell told PA: “I’m worried about the nature of the extremist politics that’s developing in this country and we have to be careful for that.
“And we have to be careful in the way individual politicians speak that may enable that to happen, and I caution the Prime Minister to that effect.”