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.

It is not viable for Venezuelan opposition leader Edmundo Gonzalez to enter the country, his ally Maria Corina Machado said on Friday, shortly after President Nicolas Maduro was sworn in for a third term despite months of controversy over the country's contested July election.

Maria Corina Machado, a 57-year-old engineer with rock-star appeal, became the fearless figurehead of Venezuela's opposition after being barred from challenging strongman Nicolas Maduro in elections last July.