Opponents of the government have begun building up in the streets of Caracas in a major test of strength for the government. President Maduro has accused protesters of using violence to pave the way for a coup.
The march - which the organizers have dubbed the "Taking of Caracas" - aims to pressure the country's electoral authorities to allow a recall referendum against President Nicolas Maduro this year.
"All of Venezuela is mobilizing for the right to vote," said Jesus Torrealba, the head of the main opposition coalition, the Democratic Unity Roundtable (MUD).
He called it "the most important political mobilization of our recent history" and vowed marchers would defy the government's "strategy of fear, blackmail and intimidation."
"It's going to fall, it's going to fall, the government is going to fall," many protesters carrying Venezuelan flags chanted.
Among those taking part in the march - which organizers said would draw 1 million people - were 100 members of the Piaroa and Jiwi indigenous tribes who traveled more than 375 miles (600 kilometers) from the Amazon rainforest.
Reaction precedes action
Maduro's government has recently jailed several prominent activists, deployed security forces across the city and warned of bloodshed. A rival pro-government rally was also planned in the center of the city.
Maduro has repeatedly accused his opponents of plotting violence during the march to pave the way for a coup such as the one that briefly toppled his late predecessor Hugo Chavez in 2002. He didn't say who he believed was behind the alleged coup plan.
"If they're coming with coups, ambushes and political violence, the revolutionary will provide an uncommon and overwhelming response," Maduro told supporters on Tuesday. Pro-government "Chavistas" - named after Maduro's predecessor, the late Hugo Chavez - staged rallies on Tuesday and Wednesday. They called on their supporters to "defend the revolution."
Hundreds of soldiers and police in armored cars were deployed.
Media is not the message
At least seven journalists have been denied entry into Venezuela ahead of Thursday's planned opposition protests. Reporters from the French daily Le Monde, Colombia's "Caracol" radio and television and Al-Jazeera, as well as one of the committee's own reporters were told they lacked proper government accreditation as journalists, the New York-based Committee to Protect Journalists said. The Miami Herald said its reporter, Jim Wyss, was also expelled.
jbh/kms (AFP, AP)