A study released on Thursday by EuroNatur, Riverwatch, World Wildlife Fund (WWF) and GEOTA highlighted failures by governments in and outside the bloc to protect rivers and biodiversity, and disregard of EU water protection legislation.
Europe has 21,387 hydropower plants, with another 8,779 planned, mainly in the Alps, the study showed. More than a third of the planned projects are in the Western Balkans, where several countries are EU aspirants.
"We are facing the end of free-flowing rivers in Europe and a collapse of biodiversity if we do not stop this hydropower madness," said Ulrich Eichelmann of Riverwatch.
"The EU Commission under Ursula von der Leyen and the national governments have to stop this expansion. Above all, they must put an end to subsidies for hydropower and improve river protection."
He said it was unacceptable that consumers' electricity bills finance hydropower investments and thus the destruction of Europe's lifelines.
In the Balkans, where most of the targeted rivers remain intact and fall into protected areas, there is excessive investment in hydropower, while the potential for solar power remains largely untapped, said EuroNatur CEO Gabriel Schwaderer.
At least 380 small plants -- below 10 megawatts -- have been built since 2009 in the Western Balkans, driven mainly by the availability of public financial support in the form of feed-in tariffs. They contribute just 3.6% of total power output.
Authorities and investors view hydropower as a green alternative to other sources of energy and say it could help the countries hit the renewable energy targets they must meet to join the EU.
In order to tackle freshwater biodiversity loss, the campaigners demanded all European countries adopt a "Blue New Deal" for European rivers to better protect valuable river stretches, and a pan-European river restoration programme.