Niamey - Niger's opposition coalition COPA 2016, whose candidate Hama Amadou was to face incumbent President Mahamadou Issoufou in a presidential run-off, said Tuesday that it was dropping out of a race it said was "unfair".
"The political opposition united by COPA 2016 has decided to withdraw from the electoral process under way (and) demand that its representatives withdraw from (national electoral commission) CENI," the opposition's Seini Oumarou said, complaining of "unfair treatment between the two candidates".
Ex-premier Oumarou complained of a lack of an "official announcement" of the results from the first round vote on February 21, in which Issoufou won 48.4 percent of the vote and Amadou -- who is in jail -- took 17.4 percent.
The run-off between the pair had been planned for March 20 after Issoufou fell just short of an overall majority.
Amadou had campaigned from behind bars after being arrested in November over his alleged role in a baby-trafficking scandal.
The African Union, which deployed 40 observers for the first round, said it was generally satisfied with the organisation of the vote, despite logistical glitches and delays.
But the opposition has slammed the "grotesque and cooked up results" and vowed to unite to prevent Issoufou winning a second term.
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COPA (Coalition for Political Change in Niger) called on its elected members to "cease all activities" in parliament, with Oumarou decrying the fact that Issoufou had announced the run-off by decree overnight.
A cabinet statement read on state television late Monday said that the Constitutional Court was holding back the definitive first-round results.
"There has been no official announcement by the Constitutional Court of the final results of the first round," Oumarou said.
In addition, the duration of the second-round campaign has been cut from 21 to 10 days in violation of the constitution, he complained.
COPA said Amadou had been "unfairly deprived of his freedom in violation of international standards of free, fair and democratic elections".
Amadou denies the baby trafficking allegations and has denounced the legal process against him as a ploy keep him sidelined from the elections.
He left Niger in August 2014 to escape the charges but returned from France last November, and was immediately arrested and jailed.
"COPA holds President Issoufou and the Constitutional Court solely responsible for the social and political situation in Niger," the opposition coalition said.