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.

Venezuelan President Nicolás Maduro will be sworn in for a third six-year term after a reelection widely viewed as illegitimate and as his administration grows increasingly brazen in cracking down on its opponents.

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