Climate group scales Buckingham Palace gates
Police are questioning three men and a woman after people who said they represented the Climate Siren group scaled the gates of Buckingham Palace.
Members of the group - which protests against climate change - locked themselves to the gates for four hours just after 14:00 BST on Saturday.
The group said they were calling on the Queen to follow Prince Charles's example in raising the issue.
The four were detained on suspicion of breaching Royal Parks regulations.
A Buckingham Palace spokeswoman said they would not be commenting.Letter to Queen
A spokesman for the Metropolitan Police said officers had removed four protesters with the help of a cherry picker by 18:35 BST and that they had been arrested under SOCPA (the Serious Organised Crime and Police Act) on suspicion of breaching section 4 of the Royal Parks and Other Open Spaces Regulations 1997.
The protest came the day after the Rio+20 global conference on sustainable development in Rio de Janeiro, Brazil, came to a close.
The group published an open letter to the Queen on its website, comparing climate change to "another great national struggle" of her generation.
It read: "We believe that our national security is at stake now, no less than it was during the dark days of the last Great War, albeit in a very different way.
"As you celebrate this year's Jubilee you must find it impossible not to view with consternation the dark clouds that once again are gathering to threaten the future of this island, with which your name has been so closely associated these last 60 years.
"We echo the sentiments of your son and heir, HRH the Prince of Wales."