Investigations into the December 1 vote have revealed a string of "unacceptable errors" on the part of electoral authorities.
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.
"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, warning Gambians not to take to the streets to protest his decision.
Soldiers were seen placing sandbags in strategic locations across the capital Banjul Friday, a development which triggered widespread unease among the already-spooked population, who had been panic-buying food before the vote due to fear of unrest.
As the military deployed onto the streets of the tiny nation, the US Embassy in Banjul urged the army to continue to show "respect for the rule of law and the outcome of the presidential election."
"The Gambian people have made a clear choice for change and a new start," the embassy added.
Latest official figures gave Barrow 43.29 percent of the votes in the presidential election, while Jammeh took 39.64 percent. The turnout was at 59 percent.
Opposition spokeswoman Isatou Touray criticised on social media a "violation of democracy" and called for people to "remain calm, lucid, vigilant and not retreat."
On December 2, Jammeh made a magnanimous concession speech on television and promised -- to general surprise -- a peaceful and swift handover of power to President-elect Barrow, sparking celebrations in the country.
But on Friday he pointed to errors which awarded victory to his opponent Adama Barrow with a slimmer margin than initially announced, claiming that numerous voters had not been able to cast their ballots.
"This is the most dubious election we ever had in the history of this country," he said.
"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.
Neighbouring Senegal immediately condemned the development, calling for a UN Security Council meeting on Gambia and urging Jammeh to accept the "democratic choice, freely expressed by the Gambian people" and continue a peaceful transition of power to President-elect Barrow.
The US State department called the move "reprehensible and unacceptable breach of faith with the people of The Gambia and an egregious attempt to undermine a credible election process and remain in power illegitimately."
"We call upon President Jammeh, who accepted the election results on December 2, to carry out an orderly transition of power to President-Elect Barrow in accordance with the Gambian constitution," spokesman Mark Toner said.
Jammeh's move upends the situation in Gambia, where the population had been hoping for a peaceful democratic transition after Barrow's shock victory this month ended Jammeh's more than two decades in power.
But pressure to prosecute Jammeh and top figures in his administration, who have been accused of widespread human rights violations, is one of the key challenges facing the new government.
Many Gambians had tired of their leader'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, especially the National Intelligence Agency that reported directly to Jammeh, fed growing resentment.
Barrow 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".
Dozens of opposition activists, including the leader of the United Democratic Party (UDP) had already been freed from prison on bail this week.