Guinea Bissau's ruling party has filed a petition at the Supreme Court asking it to nullify the December 29, 2019 presidential re-run.
The African Party for Guinea and Cape Vert Independence (PAIGC) Friday filed the notice, citing irregularities in the polls.
Last Wednesday, the National Electoral Commission (CNE) announced Mr Umaro Sissoco Embalo of the Movement for Democratic Alternative (Madem-G 15) as the winner. But PAIGC'S Domingos Simões Pereira said the provisional results had "irregularities, nullities and manipulations".
Supporters of Mr Embalo, the President-elect, say the allegations have no legal basis.
According to CNE, Mr Embalo won the presidential re-run with 53.55 per cent of the votes while Mr Pereira got 46.45 per cent. The re-run required a simple majority victory.
The two former prime ministers went head to head on December 29 to determine who will become Guinea Bissau's next president after the November 24 election failed to produce an outright winner.
Meanwhile, the USA embassy in Bissau said the elections were just, transparent, free and conducted in an orderly way.
In a statement, the diplomatic mission congratulated Guinea Bissau for a successful presidential re-run.