President-elect Donald Trump says Venezuela's embattled opposition leader must be protected as President Nicolas Maduro takes the oath of office for a third six-year term.

President Nicolás Maduro was sworn in for another six years on Friday, after claiming to win an election in July that many inside and outside Venezuela say he lost decisively.

Nicolás Maduro took the oath before parliament, vowing his third six-year term in office would be a "period of peace".