Climate activists dyed Venice's Grand Canal green Saturday, as countries at a UN climate conference in Brazil struggled to agree on the crucial issue of phasing out of fossil fuels.

Extinction Rebellion said its activists released an environmentally harmless dye into canals, rivers, lakes and fountains in a total of 10 Italian cities to highlight "the massive effects of climate collapse".

Greta Thunberg was present at the "Stop Ecocide" protest in Venice, where demonstrators dressed entirely in red with veils over their faces walked slowly through curious crowds of tourists, images showed.

The green dye was also poured into the Po river in Turin, the Reno river in Bologna, the Tara river in Taranto, as well as fountains in Padova and Genoa, the activist group said.

Negotiations at key UN climate talks in Brazil ran into overtime on Saturday with no agreement struck and delegates still locked in a bitter fight over whether to mention fossil fuels in the final text.

At stake at the Belem talks is securing a deal that paves the way for faster cuts to planet-warming emissions that are driving ever more extreme weather.

"The most important global summit to define international political agreements aimed at countering climate and social collapse is drawing to a close, and once again this year, Italy has been among the countries blocking the most ambitious proposals," said activist Paola as quoted by Extinction Rebellion, whose surname was not provided.