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MONROVIA, Liberia — Election officials announced on Thursday that Ellen Johnson Sirleaf, Africa’s only female president, had been re-elected by an overwhelming margin this week in a runoff vote that was marred by an opposition boycott.

Glenna Gordon/Agence France-Press

President Ellen Johnson Sirleaf, speaking to reporters on Thursday, said she would pursue a policy of reconciliation.

Mrs. Johnson Sirleaf won 90.8 percent of the vote in the low-turnout election, easily defeating Winston Tubman, a former United Nations diplomat who said he was withdrawing from the race only days before the voting over what he claimed was fraud in the first round.

Independent election observers found no evidence of serious irregularities in either the first or second rounds of voting, and Mr. Tubman’s motives for pulling out remained unclear. Both the Carter Center and monitors from Ecowas, the regional grouping of West African states, said both votes were generally free and fair. Analysts said Mrs. Johnson Sirleaf’s opponent had been expected to lose, boycott or not.

Mr. Tubman’s strategy proved provocative in a country that has been through decades of political violence. On Monday,his supporters clashed with Liberian police officers who responded to the crowd with tear gas and live ammunition, killing at least one of the protesters.

The Carter Center, calling Mr. Tubman’s claims “unsubstantiated,” said the election was “well-administered,” and it criticized Mrs. Johnson Sirleaf’s opponents for spoiling the vote.

Their “decision to boycott essentially denied the Liberian people a genuine choice within a competitive electoral process,” the center said in a statement on Thursday. Only 37.4 percent of eligible voters cast a ballot, about half the number who voted in the first round on Oct. 11, when 16 candidates were running. The low turnout and political unrest may leave the president vulnerable to claims that she does not hold a clear mandate going into her second term.

Mrs. Johnson Sirleaf came out on top in the first round of voting in October, shortly after having been named the joint winner of this year’s Nobel Peace Prize for her efforts in stabilizing a country torn by more than a decade of civil war. The Nobel announcement apparently boosted her re-election prospects, even as it was criticized by opponents as unfair interference in Liberia’s voting.

But she did not garner the more than 50 percent of the votes necessary to win outright, and so faced a runoff against Mr. Tubman, who conceded that the real draw on his ticket was his vice-presidential running mate, the former international soccer star George Weah. Both men lost to Mrs. Johnson Sirleaf in the 2005 presidential election.

In an interview on Thursday Mr. Tubman, a veteran Liberian political figure who once served as justice minister under the military dictator Samuel K. Doe, did not back down from his boycott call. Mr. Tubman, a member of the country’s American-descended ruling elite and whose family has long played a leading role here, said again that his party’s attitude toward the new government would be one of “noncooperation and nonrecognition.”

Mrs. Johnson Sirleaf, for her part, said she would pursue a policy of reconciliation.

“We are determined to make Liberia a post-conflict success story,” Mrs. Johnson Sirleaf said at a news conference, adding that she was considering giving out government posts to leaders of opposition parties. “I’m very confident that we’ll be able to reconcile.”

Mr. Tubman seemed disinclined to take her up on the offer. “I have never wanted a job from her government,” he said.

With 86 percent of precincts reporting, the Liberian elections chairwoman, Elizabeth Nelson, said Thursday that Mrs. Johnson Sirleaf had received 513,320 votes out of 565,391 tallied. Only 52,071 of the ballots counted, or 9.2 percent, had been cast for Mr. Tubman — his tally in 2005.

Mr. Tubman had urged Liberians not to vote and warned there could be violence if the runoff election proceeded as scheduled. When his supporters confronted the police on Monday with rocks and bottles, the police fired back, killing at least one. One of Mr. Tubman’s supporters said she and others had been held in a shipping container at the airfield across from Mrs. Johnson Sirleaf’s home. All 84 of the detainees were released late Tuesday.

Mr. Tubman said that while he regretted the loss of life during Monday’s confrontation, he did not regret the boycott.

Adam Nossiter contributed reporting from Dakar, Senegal.