Capriles, who officially lost to the late President Hugo Chavez protege by a razor-thin margin, accused Maduro's shaken government of being behind the violence that left at least seven people dead, including a police officer, and at least 61 others injured, including one person set on fire.
Capriles has called Maduro's win "illegitimate" and demanded a recount.
His campaign has said its calculations indicate the 40-year-old law school graduate and governor of Miranda, Venezuela's second most-populous state, actually won Sunday's special presidential election, called after Chavez died of cancer.
The Capriles campaign said the tight vote obligated the National Electoral Council to call for a hand count of all paper ballots.
Maduro -- a 50-year-old former bus driver and union activist with close connections to the Castro brothers in Cuba -- said in his Sunday night victory speech he would welcome a recount.
"We'll do it. We are not afraid," he said.
Shortly afterward, electoral council Director Vicente Diaz said the recount would be granted, "given the close electoral result and the fact that we live in a polarized country."
But Maduro backtracked Monday, saying there would be no recount, and the electoral council proclaimed Maduro the winner.
Maduro won 50.8 percent of the vote to Capriles' 49 percent, the latest government count indicates.
The tally gives Maduro a lead of some 270,000 votes out of 14.8 million cast. Some votes must still be counted, including those from Venezuelans living in other countries, who historically tend to vote for the opposition.
"But even if the government were willing to do a recount, what votes would be counted?" Maria Elena Ferrer, a Venezuelan national and political author who runs the Humanamente consulting firm in New York, told United Press International Tuesday night.
"The National Guard burned tons of paper ballots immediately after the polls closed, yet the government claims the vote was electronic," she said.
An estimated 1 million voters switched to voting against Maduro from supporting Chavez in October's election, The Wall Street Journal reported.
Chavez beat Capriles by 11 percentage points in that Oct. 7 election.
Sunday's election came less than six weeks after Chavez, 58, died at a Caracas military hospital.
The U.S. State Department said Tuesday it was not prepared to recognize Maduro as the election winner.
"We're not there," spokesman Patrick Ventrell said, noting both the Organization of American States and the European Union had concerns about certifying the election.
"Our position is that resolving these irregularities would have engendered more confidence in the Venezuelan people and the quality of this vote," he said.
Maduro said the Obama administration financed and orchestrated the post-election unrest. He vowed to use "a hard hand" against protesters.
He also said his United Socialist Party government would not let demonstrators into the center of Caracas Wednesday, as the opposition had planned.
"I've said that the march will not get to Caracas, and that's the way it's going to be," said Maduro, who is to be inaugurated Friday.
"If this violence continues, this revolution will radicalize," he said to a standing ovation at the headquarters of state oil company Petroleos de Venezuela SA.
Capriles said in a televised news conference later the Caracas march was canceled. He said he'd received information the government planned to "infiltrate" the demonstration, cause violence and then blame it on the opposition.
Capriles called on followers instead to bang pots at their homes in a traditional Venezuelan protest.
Government television stopped covering Capriles' news conference when he began discussing voting irregularities and instead showed Maduro talking to a rally, the Journal said.