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Venetians woke up on Sunday morning to an unusual sight, as a patch of water in the city's central waterway appeared to have turned fluorescent green.

Local authorities have collected water samples and opened an urgent investigation.

Speculation is rife as to what might have caused the water around the famous Rialto Bridge to change colour.

Theories range from the release of dye to a protest by environmental activists.

Italian media reported that local police were examining CCTV to determine whether the release might have been a stunt to coincide with the Volgalonga regatta taking place this weekend.

In an interview with Italian daily La Repubblica, Maurizio Vesco of the Regional Agency for Environmental Protection said that early analysis indicated that the green patch was likely caused by the release of fluorescein, a harmless dye commonly used to track the flow of water.

While this substance is not uncommon, Mr Vesco said that the usual dosage was one spoonful of dye powder - yet the size of the patch suggested that at least 1kg had been dumped in the waters. "I find it hard to believe that it was an incident... and that a kilo of fluorescein was casually released into the canal," he told La Repubblica.

Many social media users said the images coming out of Venice were remindful of the 1968 stunt by Argentine artist Nicolás García Uriburu, who dyed the waters of the Grand Canal green in order to raise awareness of ecological issues.