Activists turned rivers bright green in Spain's northern Basque Country at the weekend to protest a high-speed rail project, the regional government said Monday.

In an apparently coordinated action, the protesters dropped a non-toxic substance, fluorescein, into Basque river channels on Sunday morning, a spokesman for the Basque interior ministry said.

The dye briefly affected a stretch of the estuary in the Basque economic centre of Bilbao, where the rivers Ibaizabal and Nervion meet, and numerous other rivers in the Basque Country.

He said a group opposed to the construction of a high-speed rail system in the region was responsible.

The newspaper El Mundo said a group called "Mugitu!" ("Move" in the Basque language) had claimed responsibility for the action and denounced "the impact caused by the high-speed train on rivers, streams and water channels."

The high-speed TAV rail system, scheduled for completion in 2013, will link the port of Bilbao with the Basque regional capital Vitoria and the city of San Sebastian near the French border.

But some environmentalists and leftist groups have condemned the project, while the armed Basque separatist group ETA has also declared it a target, believing opposition to the system will win it support.

In December 2008, suspected ETA gunmen killed a 71-year-old businessman, Inaxio Uria, whose company was involved in the project.