"We have the full force of the state against us," Carles Puigdemont told a meeting of party officials in Barcelona.
"Faced with judicial proceedings and threats... the regional government is more determined than ever" to hold the plebiscite as planned on October 1, he said.
After sparking Spain's deepest political crisis in 40 years this week by voting to push ahead with the referendum, Catalonia's separatist-controlled regional parliament upped the ante by passing a bill early Friday outlining a transition to a possible independent republic.
Spanish Prime Minister Mariano Rajoy shot back at the Catalan government's plans.
"There will not be a referendum," Rajoy told officials from his conservative Party Popular on Saturday.
"It's my duty to preserve national unity," the premier said, adding that the laws passed by the regional government paving the way towards a referendum "illegal and anti-democratic."
Puigdemont, a lifelong proponent on independence for the region in northeast Spain, is hoping to mobilise supporters in a show of legitimacy in the face of Madrid's threats to halt the vote by any means possible.
He urged supporters of independence to take to the streets on Monday -- the region's national holiday.
"On Monday we will overwhelm them peacefully and democratically, as always," he said.
Opinion polls show that Catalans are evenly divided on independence. But over 70 percent want a referendum to take place to settle the matter, similar to the plebiscite held in Scotland in 2014.
The Catalan government staged a symbolic independence referendum in 2014, when more than 80 percent of participants voted to split from Spain -- though only 2.3 million of Catalonia's 5.4 million eligible voters took part.