Extinction Rebellion activists gather in Trafalgar Square to oppose a ban on their "autumn uprising" protests in London on Wednesday.

Visitors to a London portrait gallery were treated to an impromptu performance on Sunday when three semi-naked climate activists were doused with mock crude oil in a protest against arts sponsorship by British oil company BP.

The unauthorized stunt at the National Portrait Gallery capped a two-week wave of actions by civil disobedience movement Extinction Rebellion, which led to more than 3,300 arrests in London and other cities, the group said.

Activists spread a white sheet over the floor of the main hall of the gallery's Ondaatje wing before two women and a man wearing only skin-colored underwear adopted fetal positions on the covering.

""With compassion, we ask you to change," said an activist.