Thousands of environmental activists marched through the streets of Santiago Chile Wednesday (April 11) before the protest descended into chaos as police fired water cannons at the crowd.

The activists were protesting a Supreme Court decision giving the green light to the controversial mega dam HidroAysen.

The activists blocked downtown traffic and riot police rolled out water tanks to disperse the crowd.

Protesters set tyres on fire to block streets before police used the water tanks to put out the flames.

The $3.5 billion project is a private venture between two of Chile's largest energy companies.

Environmental studies conducted have indicated that the project would have a major impact on the environment.