CNN  — 

When Venezuelan President Nicolas Maduro secured another six-year term last May in an election widely seen as sham, the former Caracas bus driver declared himself “a better-prepared president and human being right now.”

But two weeks after being sworn in, the anointed successor of the late socialist firebrand Hugo Chavez is facing the biggest challenge to his turbulent political career.

A relative newcomer, opposition leader Juan Guaido, this week declared himself Venezuela’s acting president on a day of massive anti-government protests.

Several nations – including the United States and Canada – have officially recognized his presidency. Others, including China and Russia, still support Maduro.

A defiant Maduro appeared before a crowd of supporters outside the Miraflores presidential palace Wednesday to declare, “We will not surrender.”

President Nicolas Maduro, left, and opposition leader Juan Guaido.