The regional environmental protection agency has received samples of the altered waters and is working to identify the substance that changed their colour, the department said in a tweet.

The Venice prefect has called an emergency meeting of police forces to understand what happened and study possible countermeasures.

Sunday’s incident echoes recent episodes in Italy where environmental groups have been colouring monuments, including using vegetable charcoal to turn the waters of Rome’s Trevi fountain black in a protest against fossil fuels.

However, unlike previous cases, no activist group has come forward to claim responsibility for what happened in Venice.

Veneto region governor, Luca Zaia, posted a photo of the liquid that spread through the water near the arched Rialto Bridge. The patch was reported by residents.

According to La Repubblica newspaper, the green substance is harmless and could be one of the liquids used by experts to trace water leaks.

Italian media recalled late Argentine artist Nicolas Garcia Uriburu who poured a glowing green liquid into the Grand Canal in 1968 to promote ecological awareness during the Venice Biennale.

-AAP