Maduro has called a July 30 vote to elect members of a special assembly to retool Venezuela's 1999 constitution. The opposition says the vote is structured to pack the constitutional assembly with government supporters and allow Maduro to eliminate the few remaining checks on his power, creating a Cuba-style system dominated by his socialist party.

Maduro and the military dominate most state institutions, but the opposition controls the congress and holds three of 23 governorships. The country's chief prosecutor has recently broken with the ruling party.

Symbolic referendum

"This fraudulent constitutional assembly will create a majority that will shut congress, throw democracy out the window, wipe out state governors and fire the chief prosecutor," said former Bolivian President Jorge Quiroga, who flew to Caracas Saturday with a group of former Latin American presidents to support the opposition vote. "Tomorrow, democracy and freedom are in play."