Pro and anti-Brexit protests have taken place on Parliament Square.
About 200 people joined a pro-Brexit demonstration organised by the Democratic Football Lads Alliance (DFLA) on Saturday morning.
When members of the anti-Brexit group March for Change began to set up for a rally of their own, some members of the DFLA approached and began shouting.
A beer can was thrown towards about 10 people from the March for Change protest, before police on horseback intervened.
Police knew about both protests prior to Saturday.
A blimp in the shape of Boris Johnson had been set to fly at the March for Change, scheduled to begin in London at 2pm.
However after the altercation with the DFLA, March for Change organisers told the PA news agency the blimp would not fly as it was likely to become a target.
Tom Brufatto, one of the directors of the March for Change, said: “We are here to demonstrate peacefully and defend our democracy.
“We have been attacked three times. We respect people’s right to protest but we do not respect people’s rights to intimidate and be aggressive.
“It’s very disappointing that because of the aggression we have received today even before getting started, we have had to send the Boris blimp home because we feared it was going to be used as an excuse for violence against our supporters.
“We hope that despite the disruption that we are still able to prove an interesting and big event for our supporters.”
The March for Change event is set to feature several speakers, including MP Anna Soubry and Peter Tatchell.
Across the UK and Europe, 35 other events are being held at the same time, including a pro-democracy protest in Berlin.