Investigations into the December 1 vote have revealed a string of "unacceptable errors" on the part of electoral authorities, Jammeh said in a speech broadcast on state television, adding that he would no longer concede to opponent Adama Barrow.
Gambian President-elect Adama Barrow on Saturday called on longtime leader Yahya Jammeh to drop his challenge to last week's election results following a dramatic political U-turn that has prompted fears for the West African country.
A week after conceding defeat, Jammeh on Friday declared he no longer accepted the results of the December 1 vote, upending hopes for a peaceful political transition after his 22 years in power.
Signs of a massive security ramp-up multiplied across the capital Banjul on Saturday, while the United Nations Security Council led calls for Jammeh to stand down and eschew violence.
In a speech broadcast late Friday, Jammeh, 51, cited "unacceptable errors" by election authorities and called for new polls.
"In the same way that I accepted the results faithfully believing that the Independent Electoral Commission was independent and honest and reliable, I hereby reject the results in totality," he said.
"Let me repeat: I will not accept the results based on what has happened," he added.
Jammeh, a devout Muslim who seized power in 1994 in the former British colony, warned Gambians not to take to the streets to protest his decision.
Latest official figures gave Barrow 43.29 percent of the votes in the presidential election, while Jammeh took 39.64 percent. The turnout was 59 percent.
Those figures reflect a correction issued Monday by election authorities, showing a slimmer-than-thought victory for Barrow, of just over 19,000 votes.
Jammeh pointed to that polling error, claiming that numerous voters had not been able to cast their ballots.
"We will go back to the polls because I want to make sure every Gambian votes under an electoral commission that is impartial, independent, neutral and free from foreign influence," he said.
Barrow, a consensus candidate backed by a coalition of opposition groups, urged Jammeh to accept defeat and argued his turnaround had no legal standing.
"I wish to inform you that the outgoing president has no constitutional authority to reject the results of the elections and order for fresh elections to be held," Barrow told journalists after an opposition meeting at his home.
"I urge him to change his current position and accept the verdict of the people in good faith for the sake of The Gambia, our homeland."
Barrow also appealed to his own supporters to act with "discipline and maturity."
UN, US and other foreign authorities lambasted Jammeh and urged Gambians to keep the peace.
The UN Security Council demanded Jammeh hand over power, while the US State Department called his change of heart "an egregious attempt to undermine a credible election process and remain in power illegitimately."
EU foreign affairs head Federica Mogherini blasted the move as "unacceptable".
The West African regional grouping ECOWAS, the African Union and the UN earlier issued a joint statement urging Gambians including armed forces to "reject violence and peacefully uphold the will of the people as expressed through the ballot box".
Liberian President Ellen Johnson Sirleaf, who was leading an ECOWAS mission, was turned away while trying to travel to The Gambia Saturday.
Senegalese Foreign Minister Mankeur Ndiaye said it was Jammeh who had "prevented the landing" of Sirleaf's plane.
Jammeh's swift concession of defeat on December 2 had stunned observers and led to celebrations in the country.
Though stable under Jammeh's rule, the tiny country of just 1.99 million people faces daunting problems, including endemic poverty.
Many Gambians had tired of Jammeh's unpredictable behaviour, including the declaration of an Islamic republic in a country with a history of religious tolerance, and its withdrawal from the Commonwealth and the International Criminal Court.
The perception that Jammeh simply took over businesses and properties for his personal gain also angered many, while police harassment and impunity by the security services fed growing resentment.
With soldiers out in force, and on the eve of a Muslim holiday celebrating the Prophet Mohammed's birthday, the streets of Banjul were calm and foreigners largely out of sight.
On Thursday the president-elect Barrow announced he had the backing of the army chief. But in an apparent attempt to keep the loyalty of the armed forces, Jammeh handed out promotions to some 250 officers on Thursday and Friday.
One street juice seller, fearing a knock-on effect on his business, said Jammeh "should not say things that will scare tourists away" and hoped he would "step down peacefully for the sake of Gambians".
Pressure to prosecute Jammeh and top figures in his administration, who have been accused of widespread human rights violations, had been one of the key challenges for Barrow's incoming government.
The president-elect had vowed to set up a South Africa-style truth commission but ruled out a political "witch hunt" and promised that his predecessor would be able to "live in Gambia like any ordinary citizen".