Catalan MPs passed the motion by 70 votes to 10 -- in a ballot boycotted by the opposition -- in a move that threatens the stability of Spain and the European Union.
Madrid is now poised to impose direct rule on the semi-autonomous region after vowing to "restore legality".
Here is some of the reactions:
EU: 'Argument not force'
European Council President Donald Tusk said Madrid "remains our only interlocutor" following the independence vote.
"I hope the Spanish government favours force of argument, not argument of force," he tweeted.
European Commission chief Jean-Claude Juncker also warned of the danger of "more cracks" opening up in the 28-member bloc.
"We shouldn't insert ourselves into what is an internal debate for Spain, but I wouldn't want the European Union to consist of 95 member states in the future," he told journalists during a trip to French Guiana.
Brussels has for weeks insisted the standoff in Catalonia is an internal matter for key EU member Spain, resisting Catalan efforts to internationalise the issue and backing Madrid's position that the referendum was illegal.
US: 'Keep Spain strong and united'
The United States considers Catalonia an "integral part of Spain" and supports Madrid's measures to keep Spain "strong and united," the State Department said.
"The United States enjoys a great friendship and an enduring partnership with our NATO ally Spain," spokeswoman Heather Nauert said in a statement.
Germany: 'Dialogue and de-escalation'
Germany "does not recognise" Catalonia's unilateral declaration of independence, a government spokesman said, calling for dialogue between the two sides.
"The sovereignty and territorial integrity of Spain are and always will be inviolable," spokesman Steffen Seibert said on Twitter.
"We hope that those involved will use all available opportunities for dialogue and de-escalation."
France: 'one partner in Spain'
French President Emmanuel Macron said Spain's Prime Minister Mariano Rajoy had his full support.
"I have one partner in Spain, that's Prime Minister Rajoy... the rule of law prevails in Spain, with constitutional rules. He wants to ensure they are respected and he has my full support," Macron told reporters during a visit to French Guiana.
UK: 'Unity preserved'
Britain "does not and will not" recognise the unilateral declaration of independence, Prime Minister Theresa May's spokesman said.
"It is based on a vote that was declared illegal by the Spanish courts. We continue to want to see the rule of law upheld, the Spanish Constitution respected, and Spanish unity preserved."
However, a minister in Scotland's independence-minded devolved government said it respected the Catalan government action.
"While Spain has the right to oppose independence, the people of Catalonia must have the ability to determine their own future." said Fiona Hyslop, Scottish Cabinet Secretary for External Affairs.
"Today’s declaration of independence came about only after repeated calls for dialogue were refused."
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