In a shock rejection of his defeat at the polls, Jammeh cited in a speech broadcast late Friday "unacceptable errors" by election authorities.
Veteran Gambian leader Yahya Jammeh announced a legal challenge to an election in which he had already conceded defeat, in a weekend of dramatic twists that sparked international furore and fear of a crackdown in the west African state.
A week after conceding defeat to upstart rival Adama Barrow in the December 1 vote, Jammeh rejected the polls on Friday, citing "unacceptable errors" by election authorities.
His ruling party on Saturday qualified Jammeh's declaration as being "a prelude to a petition that the APRC party is in the process of filing before the Supreme Court".
An avalanche of international criticism rained down on immediately Jammeh, who has led the tiny sliver of a nation of two million for 22 years since taking over in a coup.
The UN Security Council unanimously demanded he hand over power, and "strongly condemned" the about-face.
Liberian President Ellen Johnson Sirleaf, who is also the chairwoman of Economic Community of West African States (ECOWAS) said Jammeh's reversal "threatened peace".
She issued a plea in a statement late Saturday for Jammeh to "do the right thing and take actions to facilitate a smooth and peaceful transition in The Gambia."
Sirleaf's plane was apparently refused the right to enter The Gambia as a mediator over the weekend.
Security forces on Sunday continued to pile into the capital Banjul where locals weighed the possibility of protest against fear of a crackdown.
Jammeh's swift concession of defeat had initially stunned observers and led to celebrations across the country.
A readjustment of the votes counted in the December 1 presidential election was made on Monday, reducing vote numbers for all three candidates but ultimately assuring Barrow's victory.
In his speech on Friday, Jammeh said he would now "reject the results in totality," and called for new elections.
One senior member of the Gambian legal community told AFP: "the president doesn't have the power to nullify the election."
By Sunday afternoon on the edges of the strategic highway that leads to Banjul, sandbags were being piled high as soldiers dug trenches out behind them.
Troops armed with assault rifles were stationed at some of these makeshift positions, eyeing the road leading to Jammeh's Banjul stronghold.
On the city's outskirts, joy had given way to nervousness as more troops watched citizens prepare for a Muslim religious holiday.
"We don't feel good; we don't feel happy. We have a new president," said Ibrahim Sisay, a businessman visiting the market in the populous Serekunda district.
Sisay said an international intervention would be welcomed by many Gambians.
"We want anyone to come. Senegal are the closest people to us but anyone, even the Japanese!" he told AFP.
Some were discussing protests while others said the cost could be heavy.
"People are afraid. If we go out on the streets he could kill us," said Assan Njie, a delivery man.
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 the correction issued Monday by election authorities, showing a slimmer-than-thought victory for Barrow, of just over 19,000 votes.
Constitutionally Jammeh has 10 days after the election result is declared to file a complaint, but that deadline -- December 12 -- is a bank holiday, meaning he may have an extra day in lieu.
His lawyers might also argue he has 10 days after the recount declaration, according to legal experts.
The Supreme Court has not sat in more than a year, so judges will have to be appointed before they can consider the president's legal complaint, potentially delaying Barrow's inauguration.
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, raising concerns from rights groups.
Barrow on Saturday called on Jammeh to drop his challenge, while the UN, US and other foreign authorities lambasted the leader and urged Gambians to keep the peace.