Bottles were thrown at officers as the protests in Trafalgar Square escalated after police began to break up huge crowds.
Scuffles broke out between demonstrators and police at an anti-vaccination and anti-lockdown protest at London’s Trafalgar Square.
From Bristol to Edinburgh, thousands of people joined protests across the UK.
Protesters across the United States were met with an increasingly heavy-handed police response, with at least 25 cities imposing curfews.
Climate protest group has vowed to blockade "every single road" in Westminster.
Police were aware both protests had been planned for central London on Saturday.
It comes amid three months of unrest in Hong Kong.
The Prime Minister's decision to prorogue parliament earlier this week has sparked widespread outrage.
Beijing and the Chinese army have issued stern warnings about the unrest in Hong Kong, where campaigners are demonstrating for the ninth weekend in a row.
The demonstration in the centre of the city lasted more than seven hours on Saturday.