Venice's Grand Canal has turned bright green but no one knows who's behind it

Police in Venice are investigating a phosphorescent green liquid patch in the city’s famed Grand Canal.

They have identified that the liquid used was fluorescein, a contrast agent most commonly used for diagnosing eye problems.

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

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 first was reported by local residents.

Police were looking into whether the green colouring could be a protest by climate activists however they have confirmed they were not behind it.

Watch the video above to see the bright green Grand Canal.