The source of a phosphorescent green liquid that emerged in Venice's Grand Canal on Sunday is being investigated by police, who claim it was caused by climate activists.

Tourists snapped photos of the green patch from the Rialto Bridge up and along part of the canal while gondoliers punted through the phosphorescent waters.

The color was discovered by locals, according to the Veneto region's president, Luca Zaia. "The prefect has called an urgent meeting with the police to investigate the origin of the liquid," he explained.

The Italian fire department said that it was assisting the area environmental protection agency in collecting samples for examination.

According to the local daily La Nuova Venezia, police were investigating if the green coloring was a protest by climate activists.

This coloring is not a stranger to Venice, as, during the 34th Venice Biennale in 1968, Argentine artist Nicolás Garcia Uriburu painted the waters green with a fluorescent dye in order to raise environmental awareness.