Why has Venice’s canal turned ‘green’?

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

A stretch of Venice's Grand Canal turned bright green Sunday, prompting police to investigate amid speculation about a stunt by environmentalists.

In this image released by the Italian firefighters, a firefighter on a boat looks at the arched Rialto Bridge along Venice's historical Grand Canal as a patch of phosphorescent green liquid spreads in it.(AP)

Gondoliers could be seen punting through the phosphorescent waters, while tourists took photographs of the green area, from the Rialto Bridge up and along part of the Canal.

The colour was first spotted by local residents, the Veneto region's president Luca Zaia said on Twitter.

"The prefect has called an urgent meeting with the police to investigate the origin of the liquid," he said.

The Italian fire service said it was helping the regional environmental protection agency take samples for testing.

It is not the first time the Grand Canal has been turned green.

In 1968, Argentine artist Nicolas Garcia Uriburu dyed the waters of Venice's Grand Canal green with a fluorescent dye during the 34th Venice Biennale in a stunt to promote ecological awareness.

Police were looking into whether Sunday's action could be a protest by climate change activists, local daily La Nuova Venezia said.