The protesters from the Extinction Rebellion group, dangling from the Rialto Bridge over the canal with the aid of climbing ropes, also displayed a banner that read: "COP28: While the government talks, we are hanging by a thread."

Small stretches of rivers and canals in Italian cities, from the Po in Turin in the north to Rome's Tiber in the south, were also turned green in similar protests.

"In a few hours, these waters will be back to what they were before," Extinction Rebellion said.

"In the meantime, while governments talk, we count the damage and the victims from constant floods and fires," it added in a statement.

The group said that the visual effect was created by a fluorescein dye that was harmless and used in industry to trace items in water.

However, Venice Mayor Luigi Brugnaro denounced what he called "eco-vandals" and called on the Italian authorities to punish them.

Boat traffic on the Grand Canal was halted on Saturday during the protest, and the canal water and the recently restored columns on the Rialto Bridge had to be checked on safety grounds, the mayor said.

"Venice is a fragile city, one that is to be loved and respected! Enough is enough," he added.

(Writing by Keith Weir; Editing by Conor Humphries)