Henrique Capriles Radonski demanded details of an audit of the vote the electoral council says it will carry out.
He said the council had a “deadline” of Thursday, but did not specify what action he would take.
Nicolas Maduro won the April 14 election by less than two percentage points.
He was sworn in as president last week, succeeding his mentor Hugo Chavez, who died in March of cancer.
But the opposition cried foul, and tensions in the divided country have reached fever pitch, with the government accusing the opposition of fomenting coup attempts and the opposition accusing the government of “desperate lies”.
Nine people died in post-election protests and both the government and opposition are planning more protests on 1st of May.
Henrique Capriles says the vote was marred by thousands of irregularities, including voter intimidation, and has demanded a full recount.
The national electoral council (CNE) offered an electronic audit of the vote last week, to begin this week, but says Nicolas Maduro’s victory remains “irreversible”.
It has so far failed to give any details of the audit and on Wednesday Henrique Capriles said he would wait only until Thursday.
“We will not accept a joke audit,” Henrique Capriles said at a news conference.
“It’s time to get serious.”
Henrique Capriles repeated his accusations that Nicolas Maduro had manipulated poll results, telling a news conference: “The truth – and it is as big as our country is wide – is that you stole the election. That is the truth.
“You stole this electoral process, and you have to explain that to this country and to the world.”
The government, meanwhile, accuses the opposition of stirring up the post-election violence in a bid to engender a coup, and the government-controlled National Assembly has now announced a commission to investigate whether Henrique Capriles was responsible.
Pedro Carreno, who will head the commission, dubbed Henrique Capriles a “murderer” as he announced its formation – joining the National Assembly head Diosdado Cabello, who has called him a “fascist murderer”.
Prisons Minister Iris Varela, meanwhile, has said a jail cell awaits Henrique Capriles.
Media coverage of the post-election violence has been at odds, with state media describing pro-opposition mobs torching health clinics but opposition media saying many reports of the violence were fabricated.
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