"We have to act now," Maria Corina Machado said in a virtual meeting with other opposition activists. "This December 1 will be a unique demonstration."
Protests will take place "inside and outside Venezuela," added Machado, who has been in hiding after government threats of imprisonment.
She also promised "firmer, more decisive action" ahead of January 10, when Maduro is set to be sworn in to a new term in office.
Machado and her party's candidate in the July 28 elections, Edmundo Gonzalez Urrutia, have accused the Maduro government of widespread electoral fraud.
The country's National Electoral Council proclaimed Maduro the winner, but the government has refused to provide detailed voting data to substantiate its claim.
Anti-government protests since then claimed 28 lives, leaving 200 people injured and more than 2,400 in jail.
The authorities subsequently launched investigations of opposition leaders and called for Gonzalez Urrutia's arrest.
He fled to Spain but has promised to return to be sworn in as president on January 10. "There is no doubt about that," he said in the virtual meeting.
"We are fighting, we are taking our voice -- the voice of all Venezuelans -- abroad," he said, adding that he had found "great receptivity" to the opposition cause during visits to Portugal, Italy and Belgium.
Countries including the United States, Italy and Ecuador have recognized Gonzalez Urrutia as Venezuela's legitimate president-elect.
