Venezuelan presidential candidate Edmundo Gonzalez said Monday the opposition had evidence to prove that he won Sunday's election.
Electoral authorities had named President Nicolas Maduro as the winner, triggering protests in the Venezuelan capital, Caracas.
Police fired tear gas to disperse thousands of protesters, while Maduro said his government "knows how to confront this situation and defeat those who are violent."
Maduro's campaign manager accused the opposition of stoking violence and called on the president's supporters to march on Tuesday.
What did the opposition say?
"I speak to you with the calmness of the truth," Gonzalez said. "We have in our hands the tally sheets that demonstrate our victory."
Opposition leader Maria Corina Machado, who was barred from running in the election, said the opposition's tallies showed a total of 2.75 million votes for Maduro and 6.27 million for Gonzalez.
The figures represent a stark difference compared with those announced by the electoral authority: 5.15 million votes for Maduro and 4.45 million for Gonzalez.
The electoral authority is facing increasing pressure to release the tallies, with independent pollsters calling the official result implausible.
Ahead of the election, pollsters had predicted a landslide victory for the opposition.
What has been the international reaction?
The US and Brazil were among the American governments that called for transparency, while others rejected the results.
Peru's Foreign Ministry ordered Venezuelan diplomats to leave the country within 72 hours, citing the "serious and arbitrary decisions made today by the Venezuelan regime."
Uruguay's Foreign Minister Omar Paganini said in remarks to broadcaster CNN that his government would "never" recognize Maduro as the winner, adding that the opposition had clearly won.
Meanwhile, 12 member nations of the Organization of American States (OAS) bloc requested a meeting over the situation.