Venezuelan opposition candidate Henrique Capriles has vowed to challenge President Nicolas Maduro's narrow victory in the 14 April presidential poll in the Supreme Court.
Mr Capriles announced on Thursday his movement would pull out of an audit of the vote, which he agreed on last week.
He said the proposed conditions for the audit were unfair, calling the process a "joke".
The opposition leader claims votes were "stolen" by Mr Maduro's government.
Mr Capriles said his movement would boycott the audit because the electoral authorities had not accepted his demand for voter signatures and fingerprints to be examined.
The electoral council failed to meet his "deadline" of 25 April to issue details of the audit.
"If we don't have access to those notebooks, we're not going to take part in an audit that would be a joke on Venezuelans and a joke on the world," Mr Capriles told pro-opposition TV channel Globovision.
Mr Capriles says he will ask the Supreme Court to annul the election and allow Venezuelans to go back to the polls.
Mr Maduro won the 14 April 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.
Nine people died in post-election protests and both the government and opposition are planning more protests on 1 May.
Mr 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 Mr Maduro's victory remains "irreversible".
"We will not accept a joke audit... It's time to get serious," Mr Capriles said at a news conference.
He repeated his accusations that Mr Maduro had manipulated poll results: "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 Mr Capriles was responsible.
Pedro Carreno, who will head the commission, dubbed Mr 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 Mr 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.