President-elect Donald Trump gave Venezuelan President Nicolas Maduro a stern warning on Thursday not to harm his country’s opposition leader. 

The president-elect issued the message after reports surfaced that Maria Corina Machado, the head of Venezuela’s opposition party and fierce critic of the South American country’s president, had been detained by Maduro’s regime. Machado was in hiding in Venezuela but made her first public appearance in weeks to lead demonstrations protesting Maduro’s presidential inauguration scheduled for Jan. 10.