"I have one counterpart in Spain, the is Prime Minister Rajoy," Macron said on a visit to the French south American territory of Guiana. "The rule of law prevails in Spain, with constitutional rules. He wants to ensure that they are respected and he has my full support."
As Catalan separatists and Rajoy's government squared up over the last few days, Macron made his support for Madrid clear on several occasions, at one point denouncing "a form of irredentism going on in Catalonia".
Foreign Affairs Minister Jean-Yves Le Drian responded to the Generalitat's move with a declaration that France "does not recognise the declaration of independence".
Spain's constitution "must be respected," he added. "France wants a strong and unified Spain."
Call for new vote
Former prime minister Manuel Valls, who was born in Barcelona, and Paris mayor Anne Hidalgo, who is of Spanish origin, also joined the chorus.
"We cannot accept the power grab of the Catalan independentists," Valls tweeted. "For Europe, for Spain, for Catalonia, we are with the Spanish government."
The independence declaration is "a serious act, against the law, which can only lead into an impasse", Hidalgo, a member of the opposition Socialist Party, declared, calling for "negotiated autonomy".
Hard-left leader Jean-Luc Mélenchon took a different line, calling for an officially recognised referendum because "things are getting out of hand".
"I don't want people declaring independence like that or people resorting to repression," he said on a visit to Greece, where he was attending the founding conference of a new party to the left of Prime Minister Alexis Tsipras's Syriza. "There should be a vote."
Rajoy was happy with the "alarming" situation, Mélenchon commented, "because it favours a sort of nationalist exultation in the rest of Spain, which he hopes to harness".
Earlier in the week, Catalan nationalists in France offered to host a government-in-exile, led by Catalan President Carles Puigdemont, in Perpignan, the main city in Pyrenées-Orientales, which was part of Catalonia before it became part of France in the 17th century.