By the BBC's Malcolm Brabant in Barcelona

More than 100,000 people brought the centre of Barcelona in Spain to a standstill on Sunday as they protested against a government plan to drain water from the country's biggest river to help the drought-ridden south.

The government says the plan, which could cost as much as 4.2bn euros, is crucial to the future development of Spain but environmentalists claim it would be a disaster.

There was a carnival atmosphere in the early spring sunshine, as a cross section of Spanish society marched from Barcelona's main square to its gothic cathedral.

Elderly ladies and middle-aged, middle-class couples sang protest songs along with trade unionists and a substantial display of opposition.

'Ecological disaster'

The government wants to pipe water from the River Ebro in the north east of the country to Mediterranean coastal areas such as Valencia, Almeria and Murcia which suffer annual shortages.

The water would be used to help agriculture and tourist areas.

The plan calls for building more than 100 dams and for some areas to be flooded.

Protestors like Max le Pretre believe it would be an ecological disaster.

"It's not sustainable from the ecological point of view," he said.

"We are not against to share the water between different autonomies in Spain.

"They will have to destroy many national parts and so on, so that's why we are against that."

Momentum

Ecologists also claim the plan could ruin parts of Catalonia and Aragon by driving young people away.

The government has sought to lessen the ecological damage by offering to reroute an aqueduct to protect wildlife.

The environment minister, Jaime Matas, claimed the proposed changes could reduce the impact by 95%.

This project has been delayed for 15 years because of widespread opposition, and the scale of this demonstration will convince opponents that they have the support and momentum to win the battle.