Mauritania's opposition parties Monday rejected the provisional results of the presidential election, won by a former general and ally of the outgoing head of state.
Saturday's election had been billed as an historic event for the conservative Saharan desert nation, marking the first democratic transition in its coup-strewn history.
But opposition parties said flaws in the vote had strengthened their fears of a power grab by the military.
"We are going to organise protest demonstrations, it's our constitutional right," Mohamed Ould Moloud, who joined three other losing candidates, told a press conference late Sunday after the CENI electoral commission published the results.
The ruling party's candidate, Mohamed Ould Ghazouani, won the presidency outright with 52 percent of the vote in the first round of voting, according to the results which must still be confirmed by the country's constitutional council.
Ghazouani easily beat main opposition opponents Biram Ould Dah Ould Abeid, an anti-slavery activist credited with 18.58 percent of the vote, followed by Sidi Mohamed Ould Boubacar, a former prime minister, with 17.87 percent, according to the official figures.
Journalist Baba Hamidou Kane and political newcomer Mohamed Lemine El-Mourteji El-Wavi, each garnered under three percent.
Mauritania's outgoing President Mohamed Ould Abdel Aziz, a general, came to power in a coup in 2008, and Ghazouani was his chief of staff for a decade.
"Multiple irregularities... have eliminated any credibility" in the elections, said Boubacar.
"We reject the results of the election and we consider that they in no way express the will of the Mauritanian people."
He vowed the opposition would use "every legal means" to challenge the outcome.
However, the four losing candidates called off a planned protest at the electoral commission headquarters on Monday.
"We have decided to postpone the march that was to have taken place today, perhaps until Thursday," Kane told AFP.
The four are demanding that CENI publish a breakdown of the vote according to each polling station, so that they can compare the figures with their own data.
CENI said voter turnout was 62.66 percent.
The 62-year-old Ghazouani, who is also former head of the domestic security service, had already declared himself the winner in the early hours of Sunday in the presence of Abdel Aziz.
Second-placed Abeid told the opposition news conference: "We are launching an appeal to the Mauritanian people... to resist, within the bounds of the law, this umpteenth coup d'etat against the will of the people."
Incidents broke out between protesters and police following Ghazouani's declaration in the capital and in northwest Nouadhibou.
The opposition is seeking the release of dozens of supporters arrested during clashes between police and protesters on Sunday.
The interior ministry also called for restraint and warned that any unauthorised assembly could lead to criminal sanctions.
Some 1.5 million people were eligible to vote in the vast predominantly Muslim state, which has a population of just 4.5 million.
Ghazouani -- who campaigned on the themes of continuity, solidarity and security -- served as Abdel Aziz's chief of staff from 2008 to last year.
Abdel Aziz, who was elected in 2009 and again in 2014 in polls boycotted by the opposition, repeatedly warned that the country could fall back into instability if his chosen candidate was not elected. He is credited with reforming the army, clamping down on jihadists and pushing to develop remote regions.
Nevertheless, rights groups have accused Mauritania's government of restrictions on freedom of expression and assembly, while calling on the nation to do more to counter violence against women and slavery, which persists in this deeply conservative state even though it was officially abolished in 1981.
The country's economic growth at 3.6 percent in 2018 has been insufficient to meet the needs of a fast-growing population, according to the World Bank.