Marches planned by Britain First and the English Defence League (EDL) through London today have clashed with a counter-protest by Unite Against Fascism (UAF).
The two far-right groups said they were protesting against radical Islamic terrorism following last month's terror attack at Westminster.
"After the vile terrorist attack on parliment [sic] we will stand together and show we will not now, not ever bow down and fear terrorists and terrorism," said a description of the event on the EDL's Facebook page.
Between the two groups, around 800 people said they were going to the events on Facebook. More than 200 signed up to go to the UAF's counter-protest, which called for unity.
Police issued protections yesterday in an effort to avoid clashes between the groups and keep each march to its own route.
According to reports, when former spokesman and leader of the EDL Tommy Robinson arrived, police swarmed to try to stop any trouble from brewing.
Video: Protesters from opposing sides have clashed during arrival of Tommy Robinson. pic.twitter.com/kARzbv6QC3— Catherine Wylie (@wyliecatherine) April 1, 2017
Police said they have have made 12 arrests for various offences.
As of 1400 hours officers deployed to the protests in #Westminster today have made 12 arrests for various offences.— MPS Events (@MetPoliceEvents) April 1, 2017
Police at the top of Whitehall attempt to contain protesters and a police medic deals with someone on the ground. pic.twitter.com/Yx2uNBI1ch— Catherine Wylie (@wyliecatherine) April 1, 2017
Yesterday, chief superintendent Catherine Roper said: "The right to protest is a fundamental right in our democratic society, but this right must be balanced against the right of people to go about their day without fear of violence, disorder or disruption.
"Experience has shown us that when groups with conflicting views come together it can create tension and disorder, not just on the day itself but in the longer term.
"If you want to protest on Saturday we ask that you do so peacefully, no matter what your view. We will adopt a robust arrest policy on anyone who attends and is intent on violence and disorder, or is in breach of these conditions."