A group of climate change activists have scaled the gates of Buckingham Palace and secured themselves to the railings.

Four protesters from the Climate Siren group locked themselves to the south centre gate just after 2pm, wearing t-shirts bearing the words "climate emergency. 10% annual emission cuts" and wielding megaphones.

Two of them held a banner carrying a quote from the Prince of Wales reading: "The doomsday clock of climate change is ticking ever faster towards midnight. We are simply not reacting quickly enough."

Other activists wearing t-shirts and carrying banners with the same message were also on the scene. The group said they were calling on the Queen to follow her son's example in speaking out about "the escalating threat of catastrophic climate change".

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 described their actions as "an act of civil disobedience designed to demand urgent, concerted and meaningful action to tackle the unprecedented national and global emergency presented by the catastrophic destabilisation of global climate and timed for the day after the end of the Rio+20 Earth Summit in Brazil."

They said they had vowed to protest against "the failure of the conference to achieve any meaningful results".

In an open letter to the Queen posted on their website, Climate Siren said their protest was to demand "a fundamental upscaling of our national effort to confront the emergency presented by catastrophic destabilisation of global climate".

A hundred or so bystanders stood and watched the protest. Activist Siobhan Grimes, 24, from Bethnal Green in east London, who was among those at the scene, said: "Our politicians aren't doing enough. We're hoping to inspire people to wake up to the consequences of catastrophic climate change."

Police sent a cherry-picker to start removing the protesters from the gate just before 6.30pm.

The activists chained to the gate were said to be Fiona Brooks, 23, Cyril Zeldine, 33, teacher Peter Coville, 45, and Phil Thornhill, all from London.