Political Demonstrations
Organisers said 430,000 people participated in the march on Sunday.
The campaigners broke into the legislative chamber and have spray painted over the walls.
The demonstration was the latest against the government's controversial extradition bill.
The rising death toll prompts UN concern that the country is sliding into a “human rights abyss”.
“We would like to thank Londoners for opening their hearts and demonstrating their willingness to act on that truth."
Greta Thunberg will address the protesters on Easter Sunday.
Protesters chanted "we are peaceful, what about you?" as officers arrived.
Over 120 activists have been arrested following protests by environmental group Extinction Rebellion. Activists blocked major London roads, including on Waterloo Bridge, Oxford Circus and Marble Arch. The group, which in early April held a naked protest in parliament, is demanding that the government declare a climate emergency.
Hundreds of protesters descended on Westminster to protest the government's perceived inability to take the country out of the European Union. As Theresa May's deal was rejected by lawmakers for a third time, Leave voters called for a no-deal Brexit.
Demonstrators gathered in front of Parliament to call for a second referendum and to protest a 'blindfold Brexit', as MPs gathered to vote on Theresa May's deal.