The protesters were expressing their discontent with proposed changes to the South American country's constitution and demanding new presidential elections.
The former union organiser says the country is the victim of an "economic war" and that opposition protests are an effort to overthrow him with US connivance, which the United States has denied.
The crowds were gathered to vote in an unofficial poll being held over concerns an official referendum set for 30 July - on whether to accept a new assembly - could herald a dictatorship.
Although he is deeply unpopular - the Datanalisis polling firm says 80 percent of Venezuelans reject Maduro as leader - he retains the loyalty of military chiefs, and can count on the support of electoral authorities and judges. Everything the opposition has tried to head off Maduro's lust for control has been stymied by the high court, which is still under Maduro's thumb.
They also fear that the constituent assembly would further weaken the National Assembly, Venezuela's opposition-controlled legislative body.
Venezuelan opposition leaders have called for a 24-hour nationwide strike to pressure the socialist government after more than seven million people rejected a plan to rewrite the constitution and consolidate the ruling party's power.
At least one person has been killed and three wounded in shooting that erupted after government supporters on motorcycles swarmed an opposition polling site in a church in the traditionally pro-government Catia neighborhood of western Caracas.
The second, organised by the government, is a rehearsal and aims to familiarise people with the constituent election due to take place on July 30.
"We want a democratic and free country", said Armando Napoles, who came out to vote.
"We are trying to prevent a civil war in our country", activist Josmar Díaz from Un Mundo Sin Mordaza told the Times of Malta of the international initiative yesterday. Fourteen million ballots were printed ahead of the referendum.
Fox said he left Venezuela as planned Monday and hadn't been personally notified of any action by the Venezuelan government. "The tribute to the fallen ones is not just to be here in this moment. However, we did not allow that beforehand and we won't allow it now", he stressed. At the start of the march, fugitive police officer Oscar Perez, who stole a helicopter and fired on government buildings on June 28, made his first public appearance since the incident.
"How does this government dare to even say 'illegitimate' when they have a process in 15 days that is completely out of the constitution", said Beatriz Olavarria.
While opponents of the government lined up to vote, Maduro loyalists held a "drill" for the constituent assembly, calling on supporters to participate in a mock voting exercise that was covered widely on state-run news outlets.