The opposition said it would challenge the result by all legal means.
Taya won two-thirds of the votes in Friday's poll while main challenger Muhammad Khuna Wild Hidala, the man he ousted in a coup to embark on his iron-fisted 19-year rule, was a distant second with 18.7%, official results showed.
During his campaign, Taya promised to deliver prosperity to Mauritania, where many hope offshore oil will bring riches to an impoverished desert nation that has a patchy human rights record.
A broad coalition, ranging from liberals to Islamists backing Hidala, joined with two other losers to denounce fraud and intimidation.
In a statement, it said it planned to file proceedings with the constitutional council to challenge what it called an "electoral hold-up".
Hidala, a former military ruler, was briefly detained on the eve of the poll on suspicion of plotting to seize power. He disappeared on Friday evening, fearing for his security.
On Saturday, he returned saying he could not leave a sinking ship and warned public protest could follow.
"We are going to use all legal means to resist this masquerade," he said.
"The coalition for a peaceful change in power declares the 2003 presidential election invalid, null and void," a Hidala aide, Lo Gourmo, told reporters.
The run-up to the poll in this coup-prone nation, where power has never changed hands at the ballot box since independence from France in 1960, was tense.
"Those in power don't want to share it. This is a pseudo democracy," said Shaikh Faad Bu Kamara, head of the Mauritanian Association of Human Rights and a sociology professor.
Taya's government has denied all charges of fraud and said the opposition was trying to deceive the 2.9 million people who live in a country that stretches from the Sahara to the Atlantic ocean, but has only about 800 km of paved roads.
Voter turnout in this election was
recorded at 60.8%
After seizing power in a 1984 coup, Taya romped home in a 1992 poll, an election held after a massive crackdown on Mauritanians suspected of plotting a coup.
He won with a massive margin in 1997 when the opposition boycotted the election.
But he angered some Arabs when he made an abrupt diplomatic about-turn, shifting support from former Iraqi President Saddam Hussein to Israel and the United States.
Five months ago, renegade soldiers tried to depose Taya, but the uprising was swiftly crushed and the president clamped down on the Islamists he blamed for the attempted putsch.
Announcing the results, the interior minister said Ahmad Wild Dadah, brother of the country's first president, was third with 6.9% and Masaud Wild Bilkhair, a son of black Africans enslaved by Arabs, had 5.0%.
Turnout was 60.8%. There were 2,258 polling stations and 1,107,400 registered voters for Friday's poll.