Thousands of Brexit protestors gathered for Saturday's Unite for Europe protest, which began at London's exclusive Park Lane next to Hyde Park before marching to parliament.
The rally went ahead amid tight security following Wednesday's deadly terror attack on Westminster, reports the Guardian.
The rally comes just four days before British Prime Minister Theresa May is due to trigger Article 50 of the Eurpean Union's Lisbon Treaty and begin the formal process of Britain leaving the bloc.
In a simultaneous anti-Brexit protest, protestors took to streets of Edinburgh organised by the city's Young European Movement.
Marchers gathered in the city centre and headed to the Scottish parliament, waving EU and Scottish flags and carrying placards reading "We want EU to stay" and "In business lying is a crime, why not in politics?"
Young European Movement Edinburgh chairman Jean Francois-Poncet said, "We want to raise the issue in British and Scottish people's lives that you have lies in the referendum campaign that people were not held accountable for and, whether you voted Remain or Leave, that is a real issue.