Protesters have clashed in London as tensions ran high between far-right groups and anti-fascist demonstrators.
A march and rally planned by Britain First and the English Defence League (EDL) - described as a "march against terrorism" - is taking place near a counter demonstration from Unite Against Fascism (UAF).
Frantic scenes unfolded when ex-EDL leader Tommy Robinson arrived and a high number of police officers moved to contain potential trouble.
Crowds gathered in Trafalgar Square from around midday and must obey conditions imposed by the Metropolitan Police which include following a particular route.
UAF protesters chanted "EDL go to hell" and hundreds of protesters spilled onto the surrounding roads.
Scotland Yard said on Friday that the information and intelligence available to the Met meant that they felt it necessary to impose the conditions to prevent the demonstrations from resulting in "serious disorder, serious damage to property, serious disruption to the life of the community, and to prevent the intimidation of local people trying to go about their business".
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.
"What we have had to carefully consider is how to balance the right to protest with the negative impact on our communities and potential violence and disorder that may have resulted from these protests going ahead as they were suggested.
"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."
On a Facebook event page, Britain First said "all patriots welcome to attend", while the EDL said on an event page: "After the vile terrorist attack on parliment we will stand together and show we will not now, not ever bow down and fear terrorists and terrorism. join us to show our strength." (sic)
The event comes just over a week after Westminster terrorist Khalid Masood's murderous rampage left four people dead.