"The moment is approaching when we will once again take to the streets of Venezuela to claim victory," said Maria Corina Machado, who went into hiding following the disputed July election.
"Let's get ready. We'll see each other in the streets in January," she said in a video published on social media, without giving details of the planned action.
Opposition candidate Edmundo Gonzalez Urrutia, who is in exile in Spain, urged Venezuelans to "commit together to upholding" his own mandate.
Demonstrations broke out across the country after Maduro was proclaimed the winner of the July 28 election, despite results published by the opposition appearing to show their candidate won by a landslide.
The protests left 28 dead, around 200 injured and more than 2,400 under arrest.
Only a handful of countries, including Venezuelan ally Russia, have recognized Maduro as the winner.
The United States and the European Parliament have recognized Gonzalez Urrutia as "president-elect."
The G7 bloc, which comprises the United States, Canada, Italy, Germany, Britain, Japan and France, said it considered Gonzalez Urrutia the election winner.
Gonzalez Urrutia, 75, fled with his wife in September after being threatened with arrest by Maduro's government.
