Three people have been arrested after orange paint was sprayed on Wellington Arch in London by Just Stop Oil protesters.
Smoke flares were also set off alongside the Grade I-listed monument, which is nearly 200 years old.
Campaigners used fire extinguishers to spray the paint as part of the protest against new fossil fuel licences.
According to the Just Stop Oil website, one of those taking action at the monument was Joshua Lane, 26, an engineer from Sheffield, who 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 endless pantomime governments."
Another of the activists, Joe Hogan, 40, from Hertfordshire, said: "Traditional, managed, sanitised forms of protest have done nothing.
"The only way forward is through sustained, disruptive civil resistance."
The protest comes just weeks after the government backed the development of the UK's largest untapped oil and gas field at Rosebank, 80 miles west of Shetland.
It came despite warnings about the climate damage of new fossil fuel projects and with analysis showing the project is likely to have little positive impact on the British economy.
The Met Police confirmed it was made aware of the protest just before 10.20am on Wednesday and three people were arrested on suspicion of criminal damage.