Residents expressed concerns after the river in their city inexplicably turned a dark blue color like pen ink.

Extraordinary footage shows the river passing through Dongguan, in Guangdong, China, coloured a deep blue as if a scene from a cartoon.

The water appeared to be lighter where the current was stronger, but reached almost black in some areas.

Many suspected the bizarre event was caused by a nearby fuel station dumping out chemical waste.

The person who filmed the video said: 'It's not special effects; I've never seen anything like it.'

He added that several hours later when he returned to check, the water in the river had returned to normal.

According to later reports, it was discovered that an employee from a nearby express delivery point had poured paint from plastic drums into the water, resulting in the colour change.

The Dongguan Ecological Environment Bureau visited the site to conduct tests, local media reported.

Further information is expected when the investigation concludes.