Chilean authorities have approved a £1.8-billion plan to dam two rivers in Patagonia for hydroelectricity, triggering angry protests and claims that swaths of pristine wilderness will be destroyed.

The HidroAysen project envisages five dams to tap the Baker and Pascua rivers, an isolated area of fjords and valleys, and generate 2.75 Giga Watts of power for Chile's booming economy.

The government has championed the dams as vital to poverty alleviation and growth, but public opinion has split, with many saying the project is unnecessary and will devastate an ecological haven.

Police arrested dozens of protesters, pictured, and clashed with hundreds more in Coihaique, a Patagonian city where on Monday a government-appointed commission voted 11 to 1 in favour of the dams after a three-year environmental review.

The commissioners were kept indoors for their own safety as people threw rocks and battled police with water cannon and tear gas. Similar scenes unfolded in the capital, Santiago.

The Patagonia Without Dams advocacy group accused the commissioners of conflicts of interest and said the project was “destructive and illegal”.

It said the dams would flood at least 5,600 hectares of rare forest ecosystems, river valleys and farmland.

An Ipsos poll found 61 per cent of Chileans opposed the dams. The polarisation offered a sharp contrast to the nation's feel-good glow after last year's rescue of 31 trapped miners, which boosted President Sebastian Pinera's ratings.

Some analysts say Chile will need to triple its energy capacity in the next 15 years to feed fast-growing industries and cities.

The council of ministers is expected to nod through the proposed dams but activists hope to win concessions in the environmental impact assessment for the next phase of the project: 1,920-km transmission lines, estimated to cost £2.3 billion, to bring electricity Santiago.

Much of the controversy hinges on whether Chile has viable alternative means to boost power capacity. With nuclear power widely considered anathema, some tout the Atacama desert as a source of immense solar thermal production, given its relative proximity to mines and industry. — © Guardian Newspapers Limited, 2011