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.

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.