Mr. Maduro was supposed to ride Mr. Chávez’s immense popularity and the wave of mourning over his death last month to a resounding victory that would ratify the leader’s idiosyncratic revolution.
Instead he squeaked by, with 51 percent of the vote, compared with 49 percent for the opposition leader,Henrique Capriles Radonski, according to an updated vote count released by electoral authorities. They reported that Mr. Maduro had gotten about 262,000 more votes out of more than 14.8 million cast.
The result is politics turned on its head in Venezuela, with an election winner who appears weakened and a loser who seems strengthened. Now, the country seems headed toward a high stakes political showdown.
The Electoral Council, which has a majority of government loyalists, certified the voting results on Monday even though Mr. Capriles, claiming he is the winner, had asked for a recount. as did the Organization for American States, through its secretary general, José Mighel Insulza.
In Washington, a State Department spokesman, Patrick Ventrell, said a recount was “an important, prudent and necessary step to ensure that all Venezuelans have confidence in these results.”
Denied the recount, Mr. Capriles called on his supporters to protest. As the
Electoral Council head, Tibisay Lucena, read the official certification, hundreds of demonstrators, most of them young people, faced off with National Guard troops in Altamira, a middle-class section of Caracas that is an opposition bastion. They burned rubbish and blocked a highway, as National Guard soldiers fired tear gas and anti-riot projectiles.
Some in the growing crowd demanded a recount. Others chanted, “Fraud! Fraud!” And as Mr. Maduro gave a speech broadcast nationally on television and radio, opponents in cities around the country banged pots and pans and honked car horns in a traditional sign of protest.
 At a news conference, Mr. Capriles said, “We believe we won the election,” although he provided no evidence. Referring to Mr. Maduro, he said, “If you go, in a cowardly way, to be certified today, you are an illegitimate president, and I say it to Venezuela and the world.”
 Government officials quickly fired back.
“We ratify to the world that yesterday there was a winner in Venezuela,” Foreign Minister Elías Jaua said in a televised news conference. “It was Nicolás Maduro.”
The powerful National Assembly president, Diosdado Cabello, on Monday rejected the possibility of a recount.
“We won, and we will insist it be respected,” he said.
Ms. Lucena also appeared to discourage a recount, defending the country’s digital voting system as bulletproof.
Digital voting machines in Venezuela print out a paper voucher that records each voter’s choice. Voters then deposit the voucher in a sealed cardboard box. When voting is finished, boxes in 54 percent of precincts are opened and the votes recorded on the vouchers are counted to verify the digital count.
Mr. Capriles is asking for a full recount of all vouchers and also insisted that the vouchers be checked against notebooks showing how many people voted at each precinct.
In his remarks after the election certification Mr. Maduro accused the opposition of trying to provoke violence.
The election results were particularly striking compared with the election in October, in which Mr. Capriles faced off against Mr. Chávez, and the ailing incumbent won by 11 percentage points.
Now Mr. Maduro’s relatively poor showing, even in victory, has made him vulnerable, exposing him to pressures from the opposition and, ultimately, from competing interests within Mr. Chávez’s movement.
“He’s a weakened president,” said Vladimir Villegas, a former ambassador in Mr. Chávez’s government. “He was on the brink of a defeat that would have been terrible for Chavismo.”
Mr. Maduro ran on his pledge to carry out Mr. Chávez’s instructions to the letter, calling himself “the son of Chávez.” But he turned out to be a poor campaigner, struggling to connect with audiences. Comparisons with his charismatic former boss were clear and unfavorable, and he squandered what had appeared to have been a large lead.
The roots of Mr. Maduro’s weak candidacy lay in part in Mr. Chávez’s form of governing, which built a cult of personality that glorified a leader who made all major decisions. Mr. Chávez never allowed other leaders in his movement to develop, perhaps out of fear that they might turn into competitors.
Mr. Chávez’s foreign minister for six years, Mr. Maduro was named vice president last October and became interim president after Mr. Chávez, who had cancer, died on March 5.
“The problem is you have a governance model that’s based on one-man rule and nobody else makes the decisions,” said Michael Shifter, president of Inter-American Dialogue, a policy group in Washington. “And then he dies and you have someone who is completely out of practice and has no political skills at all.”
(Chasity M.)