Climate activists splashed a viscous liquid over a glass case housing a replica mummy at Barcelona's Egyptian Museum on Sunday in the latest attack targeting cultural exhibits in protest at inaction over global warming.

The two activists doused the case with red and brown gunge from Coca-Cola bottles, also splattering framed images on surrounding walls, according to footage published on the Publico news website.

They then glued their hands beside a nearby exhibit and held up a modified Coca-Cola banner scrawled with the words "climate justice". 

The US drinks giant is one of the official sponsors of the UN's COP27 climate summit in Egypt, a position widely denounced by environmentalists who say the company is behind much of the world's plastic pollution. 

When police arrived, the two activists cooperated and neither was arrested, but the museum said it would file a complaint for damages. 

The protesting pair were part of Futuro Vegetal - a Spanish group whose activists glued their hands to the frames of paintings by Spanish master Francisco Goya at Madrid's Prado Museum on November 5.

Sunday's stunt was the latest in a string of climate protests by activists who have thrown soup at Vincent van Gogh paintings in London and Rome, and mashed potatoes on a Monet masterpiece.

On Thursday, dozens of the world's top museums issued a statement saying they were "deeply shaken" by these actions, adding that activists "severely underestimate" the damage they could cause. 

The Futuro Vegetal activists said their latest protest was to "highlight the emergency" the world faces.

"Currently 99% of the world's plastic is produced from fossil fuels," the group said.

"We find it hard to believe the global leader in plastic pollution is not going to use its influence as a sponsor of COP27 to advance its commercial interests given its total dependence on fossil fuels." 

The group warned: "Climate change policies for 2030 will lead us to an increase of 2.5C by 2060, which will mean an increase in the frequency and intensity of extreme weather events incompatible with the survival of most crops for human consumption, leading to crop failures and famine."

Futuro Vegetal wants Spain to end livestock subsidies and use the funds to promote plant-based alternatives.