Antonio Ledezma, 62, managed to cross the border yesterday, saying he would now help organise resistance to Maduro's increasingly authoritarian rule which has left millions facing food and medicine shortages.
From Bogota, he flew to the Spanish capital where he was met by his wife and his two daughters as well as former Colombian president Andres Pastrana.
A lawyer by profession, Ledezma was arrested and jailed in February 2015 over allegations he was plotting to overthrow the president, and had been under house arrest since August following surgery.
Ledezma said he plans to travel the world to demonstrate "the hope of all Venezuelans to finish with this regime, this dictatorship," and accused Maduro's government of "colluding with drug traffickers".
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He said yesterday he had fled Caracas because Venezuelan military and intelligence officials had informed him of a "government plan" against him, though he provided no details.
Oil-rich but cash-poor Venezuela is facing a deepening political and economic crisis as Maduro has moved to marginalise opposition forces, which control the country's legislature, and stifle independent media.