Green activists dumped horse manure and staged a mock hanging outside the venue of a UN climate summit in Madrid on Saturday, airing their frustration at the failure of world leaders to take meaningful action against global warming.

Led by grassroots group Extinction Rebellion, the actions were timed to coincide with the closing of the COP25 summit, where negotiators have been unable to agree on how to implement the 2015 Paris climate agreement.

"Just like rearranging deckchairs on the Titanic, this COP’s fiddling of carbon accounting and negotiating of Article 6 is not commensurate to the planetary emergency we face," Extinction Rebellion said in a statement.

Twelve members of the group stood on melting blocks of ice, nooses drawn tight around their necks to symbolise the 12 months remaining until the next summit, when the Paris deal enters a make-or-break implementation phase.

Attached to the pile of manure was a short message to leaders saying "the horseshit stops here."

In contrast to a protest held last weekend, in which hundreds of demonstrators blocked one of Madrid's central shopping streets for a mass disco-dance, the mood at the gathering was subdued.

"Even if they reach an agreement it's still not enough. This is the 25th COP they've had, and nothing has really changed," protester Emma Deane told Reuters from her perch atop an ice block, holding her young daughter in her arms.

"She's going to grow up in a world where there's no food on the shelves and that breaks my heart."

Still, Extinction Rebellion spokesman Ronan McNern stressed the importance of humour in the face of the climate crisis.

"Out of shit comes the best roses. We hope that the international community comes together to create a beautiful future," McNern said.

- Reuters