Chad's constitutional council confirmed Mahamat Idriss Deby as winner of the May 6 presidential election on Thursday after dismissing challenges by two losing candidates - cementing a victory that extended his family's decades-long rule.

Deby, who seized power the day rebels killed his father, President Idriss Deby, in 2021 and declared himself interim leader, won 61% of the vote, well ahead of second-placed candidate Succes Masra with 18.54%, the council said.

Opposition leader Masra acknowledged the council's ruling and did not say he would fight it.

"With the decision of the constitutional council today, we have used all available legal means, and even if we do not accept this decision, there are no other legal means in our judicial architecture," he said in a live address on Thursday evening.

The oil-producing country is the first of a string of coup-hit states in West and Central Africa's Sahel region to attempt a return to constitutional rule by holding elections.

It has remained a key Western ally in the fight against al Qaeda and Islamic State-linked insurgencies in the Sahel region, even as Niger, Mali and Burkina Faso kicked out Western forces and turned to Russia for support instead.

But Washington and former colonial power France have kept a wary eye since Chad's air force chief last month told the U.S. to halt activities at an air base, citing problems with paperwork. The U.S. announced a temporary withdrawal of at least some troops in response.

U.S. State Department spokesperson Matthew Miller in a statement said Washington was concerned that thousands of civil society and opposition party observers were barred from observing the electoral process the day before the vote.

Miller also expressed concern that the transitional government was not fully inclusive in setting up the institutions responsible for organising the elections.

"Although there were troubling shortcomings, we welcome the milestones in Chad's transition process, including negotiating with insurgents, undertaking a National Dialogue, holding a constitutional referendum, and conducting a presidential election," he said.

Masra, who was appointed prime minister of a transitional government in January, on Monday lodged an appeal with the constitutional council to challenge preliminary results released last week.

He had claimed victory before the official announcement of preliminary results, alleging that electoral fraud was being planned.

Albert Pahimini Padacke, the other candidate who challenged the preliminary results, congratulated Deby on his win.

The constitutional council ruled that both complaints had lacked sufficient proof.

"In view of the votes cast on the occasion of the presidential election of May 6, Mahamat Idriss Deby having obtained ... more than the absolute majority of votes cast, that it is appropriate to declare him president-elect of the republic," the council's president, Jean-Bernard Padare, said.

Deby's victory prolongs the rule of the family that has had a firm grip on power since Deby's father took over in a coup in the early 1990s.

At least 10 people, including children, were killed and dozens were injured by celebratory gunfire on Friday following the announcement of the preliminary results, according to Amnesty International and Chadian media.

Another opposition figure who had been expected to run, Yaya Dillo, was shot and killed in N'Djamena on Feb. 28, the day the election date was announced. The opposition has called Dillo's death an assassination and forensic experts have said he was likely shot at point-blank range.

Before the election, some opposition parties and civil society groups had called for a boycott, saying Deby and his allies control the main institutions of power, including the constitutional council, and could influence the process.

Authorities banned the publication of polling station results.