The populist right-wing leader of 12 years says it is "clear" his party has lost the vote billed as a referendum on the country's future.

Machado had left the country just a month before the US government seized Venezuelan President Nicolás Maduro.

He will be observed to evaluate whether his "psychiatric health is compatible with pursuing his interrogation", authorities said.

Serbia is heading to the polls as a student movement attempts to oust longtime populist leader Aleksandar Vučić.