European leaders and green groups reacted with anger and dismay after President Donald Trump announced that the United States, the world's second biggest carbon emitter, was quitting the 2015 Paris Agreement.
But they also pledged to defend the agreement and not to backtrack in the fight against climate change.
Jean-Claude Juncker, head of the European Union's executive Commission, lashed Trump's decision as "seriously wrong."
The body's commissioner for climate action and energy Miguel Arias Canete pledged continued "global leadership" on climate change.
"The EU deeply regrets the unilateral decision by the Trump administration," he said yesterday in a statement.
"The Paris Agreement will endure. The world can continue to count on Europe for global leadership in the fight against climate change.
"Europe will lead through ambitious climate policies and through continued support to the poor and vulnerable."
In Berlin, Chancellor Angela Merkel expressed "regret" at the decision, and called for a continuation of "climate policies which preserve our world."
Seven Social Democratic ministers in her coalition government said the United States "is harming itself, we Europeans and all the people of the world."
In France, Paris city hall said it would illuminate its building in green on Thursday "in a sign of disapproval" of Trump's announcement and to recall the determination of cities around the world to fight climate change.
In Rome, Italian Prime Minister Paolo Gentiloni said his country should not retreat from its actions on climate.
"Let's not go backwards from the Paris Agreement," he said on Twitter. "Italy is committed to reducing (carbon) emissions, to renewable energy, sustainable development."
Among environment groups, Climate Action Network, said the withdrawal "signals that the Trump Administration is in total discord with both reality and the rest of the world."
"Unfortunately, the first to suffer from this injudicious decision is the American people," the group, an alliance of climate activists, said.
"This action is totally contrary to their best interests: their health, security, food supply, jobs and future."
Friends of the Earth International said "pulling out of the Paris Agreement would make the US a rogue state on climate change. The rest of the world cannot let the US drag it down.
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