Protesters set off fireworks at Parliament Square and burned an effigy of Boris Johnson as hundreds gathered for the demonstrations.
Crowds gathered at Parliament Square and Trafalgar Square for the Million Mask March, which has been held since 2012 to protest against issues including government corruption, homelessness, and inequality.
This year’s protests also attracted anti-vaccine protesters, with chants of “you can stick your poison vaccine up your arse” heard.
Nearly 200 people were arrested at last year’s protests, which were held while the country was under Covid restrictions.
Videos of this year’s clashes showed crowds of protesters being met with a heavy police presence.
Another video showed an officer being hit on the head by a water bottle. The Met said it had removed fireworks from people in the crowd after rockets had hit people and exploded near others.
By 9pm, the Met confirmed it had issued a section 35 dispersal order for areas of Westminster, including Parliament Square.
In a statement on Twitter, the Metropolitan Police 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.”
In an updated statement made on Monday, the Met said 12 officers had been injured, although none seriously. A spokesperson told the Big Issue: “Officers made a total of 12 arrests while policing a number of demonstrations across central London on Friday, 5 November. Arrests were for a variety of offences. During the demonstrations, officers worked hard to minimise disorder and take action against those intent on causing violence.
“Twelve officers were injured during the course of the operation following clashes with crowds who were intent on causing disorder. Thankfully, none of the injuries were serious.
“Officers remained professional despite provocation and worked to prevent groups from dangerously setting off fireworks into the crowd. These fireworks could have resulted in serious injury and endangered passing members of the public. This was averted due to proactive interventions and arrests.
Urgent action is needed to prevent even more people being pushed into homelessness. A secure home is the first step in addressing the cruel cycle of poverty to ensure people can fulfil their potential. Join us to keep people in their homes.