Eyewitnesses suggested they gained access to the building from the River Thames.
Westminster has been at the centre of recent protests in previous weeks.
An anti-lockdown protester climbed on London’s Big Ben in a call to end coronavirus restrictions across the country.
One of the banners unfolded on the iconic London landmark read: “No Tier 3 lock-down for Gt N’n Pwrhse unless an xtra £2bn for whole region from Chester, Stoke, Derby, Nott’m, Boston, Skeg’s to Scottish border.
“If not: Gt N’n rebellion. XR North + unions + BLM UK + LGTBQ + many more.”
While police arrested an Extinction Rebellion protester dressed as Boris Johnson back in October after after a three hour standoff when he scaled Big Ben.
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