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

Images on social media show a bright patch of green in the canal along an embankment lined with restaurants.

Zaia said that officials had requested that the police investigate to determine who was responsible. Environmental authorities were also testing the water.

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, 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.