Two supporters of Just Stop Oil have covered the Madame Tussauds waxwork model of King Charles III with chocolate cake.
The climate activists are demanding the government halts all new oil and gas licences and consents.
Eilidh McFadden, 20, from Glasgow, and Tom Johnson, 29, a painter and decorator from Sunderland, stepped over the barrier in the London venue on Monday morning and smeared the model with cake.
In a statement the pair said: “We are here because we seek to protect our freedoms and rights, because we seek to protect this green and pleasant land which is the inheritance of us all.
“The science is clear. The demand is simple: just stop new oil and gas. It’s a piece of cake.”
The Metropolitan Police tweeted: “We responded quickly to an incident at Madame Tussauds after two people threw food at a statue at approximately 10:50hrs.
“They have both been arrested for criminal damage.”
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The force later confirmed that four people had been arrested.
Just Stop Oil said Monday was their 24th day of civil unrest.
Others have scaled the Queen Elizabeth II Bridge 200ft (60m) above the Dartford Crossing, which had to close for 36 hours and caused six-hour delays around much of the M25.