"We have to stand together in Europe," Hollande said during the meeting in Lisbon. "What is at stake is populism. The kind of discourse now coming from the United States encourages populism and even extremism."
Hollande was speaking at a meeting of southern European leaders in Lisbon, which showed growing concern of threats to European unity at a time of Britain's departure from the EU and growing populism.
The leaders, who face risks ranging from Europe's refugee crisis to slow growing economies and high debts, said in a joint declaration they confront a world with "growing uncertainties and instability," making European unity yet more important.
"Weakening Europe is not an option," they said.
The group, which is sometimes referred to as "Club Med", includes France, Italy, Spain, Portugal, Greece, Malta and Cyprus.
None of the other leaders spoke directly about U.S. President Donald Trump's administration, but they showed their determination in opposing protectionist trade policies that Trump is promoting.
Concerns that populism could further undermine the European Union after Britain's departure have focused on the threats to mainstream governments at elections this year in the Netherlands, France and Germany.