Protesters have burned down an effigy of Boris Johnson and thrown fireworks at the annual Million Mask March.
Hundreds of people joined the anti-government demonstration in central London, marching through key locations including Trafalgar Square and Parliament Square.
Rioters wearing Guy Fawkes masks set off fireworks across the capital as the Metropolitan Police attended the protest.
Police confirmed 12 people have been arrested and eight officers have been injured during the protest.
A Section 35 dispersal order remains in place for Parliament Square and areas in Westminster, the Met added.
A Metropolitan Police spokesperson said: "A total of 12 arrests were made while policing tonight's demonstrations across London. Those arrests were for a variety of offences.
"Eight of our officers were injured. This is unacceptable. The policing operation will continue into the evening."
In the late afternoon, a large group of people gathered in Trafalgar Square, where they burnt an effigy representing the Prime Minister.
Officers said they moved into the crowd after fireworks exploded near protesters, with some striking people.
The police force said protesters moved from Parliament Square along Whitehall before walking from Oxford Circus up Regents Street.
The annual protest first took place in 2012 and since then demonstrators have rallied against government corruption, inequality, homelessness, as well as lockdown restrictions.
A Met Police spokesperson said: "A crowd in Parliament Square have been dangerously lighting fireworks and rockets.
"Some have struck people or exploded near to the crowd, this could cause very serious injury.
"We have moved into the crowd to remove any fireworks and prevent people coming to harm."
Last year, almost 200 people were arrested during the protest for breaching lockdown restrictions.
Earlier today, the Met Police said it was planning a major operation ahead of the protest.
Deputy Assistant Commissioner Jane Connors said: "Residents and visitors to central London today can expect to see a higher number of police officers on duty throughout the course of the day.
"Officers are there to keep people safe and to respond quickly to any potential disorder of violence.
"Groups of course have the right to protest, but I am particularly concerned that some groups are specifically intending to travel into London to deliberately cause violence and disorder including targeting police officers.
"This will not be tolerated and our policing plan has been developed with this potential risk in mind.
"Our patrols will be highly visible and we will be working closely throughout the operation with the City of London Police and the British Transport Police."
