Thirty-two people have been arrested after hundreds of anti-vaccine and lockdown protesters descended on central London.
The “Resist And Act For Freedom” rally saw heated clashes between demonstrators and police on Saturday afternoon.
Dozens of officers, including some on horseback, were repelled by human blockades as they urged the crowds to disperse.
Scotland Yard said the hordes of people were “putting themselves and others at risk”.
He said he was “extremely concerned” about the rate of transmission in the capital, where the number of weekly cases per 100,000 people is reported to have increased from 18.8 to around 25.
The Metropolitan Police issued a statement at around lunchtime saying there had been “pockets of hostility and outbreaks of violence towards officers”.
“We will now be taking enforcement action to disperse those who remain in the area," the force warned.
"Those who remain may get arrested.”
The statement continued: “It is important to remember that we are still in the middle of a global pandemic, and the changes have been introduced to help control the spread of the virus, keep everybody safe and save lives.
“We encourage those in attendance to leave the area immediately.”
Protests are exempt from new legal restrictions introduced on Monday limiting groups to six, but only if they are “organised in compliance with Covid-19 Secure guidance”, the Government said.
One protester appeared to have a bloodied head following a scuffle, while another was seen receiving medical attention on the ground as several officers surrounded the scene.
Traffic around Trafalgar Square came to a halt during the demonstration, with one protester seen apparently spitting through the open window of a taxi whose driver had beeped the horn in frustration.
Rally organisers sold T-shirts bearing 5G conspiracy theories, with banners calling for Government scientific advisers to be sacked and declaring Covid-19 a “hoax”.
Addressing the crowd to huge cheers, organiser Kate Shemirani said: “We are the resistance.”
The protest was advertised with an image showing a vaccine bottle and urging people to “Come together, resist and act.”
One speaker at the rally, Professor Dolores Cahill of University College Dublin (UCD), suggested a coronavirus vaccine could “make people sick” – in a break with established scientific opinion.
The UCD has previously disassociated itself from views on Covid-19 aired by Prof Cahill, who also chairs the Eurosceptic Irish Freedom Party, the Irish Times reported.
Vaccines are the most effective way to prevent infectious disease and have virtually eradicated smallpox, polio and tetanus in the UK, the NHS says.
But if people stop getting vaccinated then diseases can quickly spread again, it said, pointing to a spike in measles and mumps between 2016 and 2018.
There is no evidence that vaccines cause autism, allergies or other conditions, weaken the immune system in any way, or contain harmful ingredients, it adds.
The World Health Organisation says immunisation prevents two to three million deaths per year.