Donald Trump says Venezuela's embattled opposition leader must be protected, as President Nicolas Maduro prepares to take the oath again after allegedly stealing an election.

Venezuelan strongman Nicolás Maduro began a third term as president on Friday, cementing his increasingly authoritarian rule.

President Nicolás Maduro will extend his increasingly repressive rule over Venezuela until 2031 when he is sworn in on Friday, despite credible evidence that his opponent won the latest election and following protests against his plan to serve a third six-year term.