Six people have been arrested at a St George's Day event in central London, the Metropolitan Police has said.
A man was arrested on suspicion of animal cruelty after "a police horse was targeted" in Westminster.
Three have been held on suspicion of assaulting an emergency worker, one for suspected assault and one on suspicion of being drunk and disorderly.
The Met shared a video on X which seems to show officers forming a cordon and a group pushing past it.
Whitehall had to close to traffic while the situation was resolved.
Afterwards a group gathered opposite Downing Street where there was also a large police presence.
The Metropolitan Police posted a video just after 14:00 BST on Tuesday along with the message: "The event is not due to start for an hour and regrettably officers are already dealing with disorder.
"There is an area allocated for this event in Richmond Terrace. This group went past it and continued up Whitehall.
"When officers formed a cordon and asked the group to turn round, they reacted by violently forcing their way through. Mounted officers intervened with horses to restore the cordon."
An earlier post from police said a number of people had been seen in the area wearing masks, and a Section 60AA order was in force in the boroughs of Lambeth and Westminster "giving officers the power to require the removal of face coverings".
Attendees were seen waving St George's Cross flags and listening to speeches, including by far-right activist Tommy Robinson (real name Stephen Yaxley Lennon).
