Just Stop Oil activists sprayed Wellington Arch in central London with orange paint on Wednesday morning.
Three supporters vandalised the historic arch, completed in 1827, where Hyde Park meets Green Park and brandished their logo on the bottom of the Grade I-listed structure at around 10.15am.
Eco-warriors said they were demanding the UK government immediately halt all new oil and gas projects and called on the public to march in the capital from October 29.
In a statement, the Metropolitan Police added: “Officers that were nearby immediately responded.
“Three people have been arrested on suspicion of criminal damage.”
One of those taking part was Joshua Lane, 26, an engineer from Sheffield.
He said: “I am compelled to take action due to the severity, and sheer emergency we find ourselves in today.
“Future generations live in uncertainty, and we are given false promises time after time by endlesspantomime governments.
“Humanity faces the biggest crisis in history, and yet the government continuously ignores the facts, and only thinks of themselves and the billionaire oil barons they serve.
“One thing that cannot be ignored is the power of ordinary people, and our voices are only continuing to grow louder.”
Another demonstrator Joe Hogan, 40, from Hertfordshire, said: “I’m angry. I’m anxious. I’m depressed.
“We are out of time; we have to act now. Not tomorrow, not next year, not 2050.
“Our government continues with business-as-usual, burning our futures to enrich their friends, while enacting draconian legislation to stamp out dissent. I refuse to be cowed.”
