Residents of Venice are scratching their heads after part of the city’s famed Grand Canal turned bright green at the weekend.

Veneto region governor Luca Zaia posted a photo of phosphorescent green liquid patch that spread through the water near the arched Rialto Bridge.

It was reported by residents on Sunday (local time). Images on social media show a bright patch of green in the canal along an embankment lined with restaurants.

Mr Zaia said officials had requested the police investigate to determine who was responsible.

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

The Regional Agency for Environmental Protection and Prevention of the Veneto (ARPAV) has begun analysing the substance to verify its composition. Italy’s fire department posted a video of the bright green patch to social media.

“The Grand Canal coloured in green is what the fire department found this morning as we intervened together with ARPAV to collect samples and analyse this abnormal colour,” it said.

The Venice prefect has called an emergency meeting of police forces into the issue, Italian media reported.

In another twist, Italian media recalled a stunt by late Argentine artist Nicolas Garcia Uriburu. He poured a glowing green liquid into the Grand Canal during the Venice Biennale in 1968 to promote ecological awareness.

Sunday’s incident came a week after climate protesters associated with Italian eco-activism group Ultima Generazione dyed the water in Rome’s Trevi Fountain black using diluted charcoal.

They were protesting after floods in northern Italy’s Emilia-Romagna region, which killed more than a dozen people after half the annual expected rainfall was generated in a 36-hour deluge that some scientists have linked to climate change.

In a statement, Ultima Generazione called for an end to public subsidies for fossil fuels, saying one in four houses in Italy was at risk from flooding.

The eight activists were arrested after last Sunday’s protest, which lasted 15 minutes.

Rome mayor Roberto Gualtieri condemned the protest, the latest in a series of acts targeting works of art in Italy.

“Enough of these absurd attacks on our artistic heritage,” he wrote on Twitter.

The famed fountain was to be drained, cleaned, and re-filled. There are no indications it has sustained any damage.

-with AAP