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.

Venezuela's President Nicolas Maduro, whose nearly 12 years in office have been marked by a deep economic and social crisis, was sworn in for his third term on Friday, remaining in power despite a six-month-long dispute over a contested July election and international calls for him to stand aside.

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.