The latest stunt targeting works of art came after Home Secretary Suella Braverman threatened a police clampdown against "direct-action" protests, including by the group Just Stop Oil.

On Saturday, nearly 30 demonstrators from the group glued themselves to the tarmac as they blocked a major road in east London, triggering angry exchanges with motorists.

After attacking the van Gogh painting, Just Stop Oil climate activists Anna Holland, 20, and Phoebe Plummer, 21, both pleaded not guilty in the London court to criminal damage.

The painting itself was protected by a screen but damage was caused to the frame, according to the gallery in Trafalgar Square.

District judge Tan Irkam released the two women on bail, pending a trial on December 13 in London.

Holland and Plummer also glued themselves to the gallery wall during their protest.

Another Just Stop Oil protest on Friday targeted the New Scotland Yard headquarters of London's Metropolitan Police, who arrested 28 demonstrators.

In the same London court Saturday, Lora Johnson, 38, pleaded not guilty to criminal damage after allegedly spraying orange paint on the New Scotland Yard sign, as others blocked the road outside.

At the Conservatives' annual conference this month, Braverman vowed to go after "protesters who use guerrilla tactics and bring chaos and misery to the law-abiding majority".

However, Just Stop Oil argues that climate changes poses an existential crisis for humanity and that its direct tactics are justified.

"What is worth more -- art or life?" Plummer shouted at the National Gallery on Friday.