This is the terrifying moment dozens of people are flattened by a massive wave as they gathered to watch one of the world's most powerful tidal bores.

The crowds were watching the tidal wave on Qiantang River, Zhejiang Province, eastern China, but wandered too close to the bank on the viewing platform.

Shocking video footage shows that within seconds a massive wave had swept away the riverbank defenses and sent crowds of tourists flying.

In the video, tourists are heard screaming in fear as they realize the powerful wave is about to hit. As they try to flee, the wave overtakes them throwing them into the air or sweeping them away.

When the water subsides, dozens of stricken tourists lay injured or stunned on the river bank. Local media reports that some needed hospital treatment. Among them can be seen the shattered remains of the riverbank's smashed to pieces guard rails.

One visitor named as Ms. Guo told local media: 'There were many security personnel, and the staff arrived at the scene promptly.

'They quickly moved people from the embankment to higher ground.'

A tidal bore is a tidal phenomenon in which a wave flows upstream in some rivers as high tide approaches. The Qiantang River tidal bore is famous for its size with waves that can reach up to 30 feet in height.

It usually occurs during the eighth lunar month of the Chinese calendar, when the gravitational pull of the moon is particularly strong and hundreds of people visit the river's banks to watch the rare spectacle.