Far-right protesters have arrived in London ahead of a march called by Britain First and the English Defence League "against radical Islamic terrorism".
Britain First claimed in a Facebook post that "3,000 patriots have confirmed attendance", but the number appears to be much lower.
The march was called in the aftermath of last week's Westminster terror attacks, where Khalid Masood killed four people and left dozens injured.
A counter-protest has been organised by anti-racist group Unite Against Fascism, who said in a statement:
"Both groups are notorious Islamophobes whose concern is not with the victims or their relatives, but only lie in racism."
Police have issued two Public Order Acts in order to avoid clashes between the protesters, and keep each march to its designated route.
The Metropolitan Police events team warned that those who did not adhere to the conditions would be subject to arrest and prosecution.
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."
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."
Tommy Robinson, the former leader of the English Defence League, was escorted through the streets by police on his way to the rally, after he was confronted by anti-fascist protesters.
A livestream of the event showed police officers arresting at least one protester from the Unite Against Fascism counter-demonstration.
Video footage posted to Twitter shows protesters from the opposing groups clashing, in an incident which may have led to further arrests.
The far-right march will set off from Charing Cross Station and will proceed along the Strand and the Embankment.
Britain First and EDL supporters will return to Charing Cross for a static rally, which must conclude by 3pm.
