Chad's Supreme Court said Wednesday that nine candidates including incumbent President Idriss Deby Itno had been approved to run in the country's April 11 presidential election.
The court rejected seven contenders but upheld 10 candidacies in the vote pitting a fractured opposition against Deby, who has ruled Chad with an iron fist since a 1990 coup.
But leading opposition figure Saleh Kebzabo had already said on Monday that he would withdraw from the race after a deadly shootout with security forces at the home of another candidate, Yaya Dillo Djerou.
In a letter to the Supreme Court that he forwarded to AFP late Wednesday, Kebzabo repeated that he "refused to provide cover for an electoral masquerade".
Judges rejected Yaya Dillo's own candidacy alongside a rising young opposition leader and harsh critic of the regime, Succes Masra, with the court saying their parties were not "legally established".
International rights groups, the UN and France have called for an inquiry into the shootings at Yaya Dillo's house, in which his party said five of his family died.
The government has only acknowledged the deaths of his mother and two soldiers, while Yaya Dillo himself escaped unharmed from the attempt to arrest him for alleged slander against Deby's wife.
Ndjamena has in recent months systematically banned or broken up all attempted demonstrations by the opposition and civil society groups calling for "a change in government and more social justice".
Amnesty International last month called the bans "unnecessary and disproportionate restrictions on the exercise of the right to freedom of peaceful assembly", also denouncing "arbitrary arrests".
UN Secretary General Antonio Guterres on Monday called for a "prompt and rigorous investigation", adding that he "regrets the use of violence and the subsequent loss of life" in the attempt to arrest Yaya Dillo.
With eight challengers split among genuine opponents and those seen by analysts as regime fronts, Deby appears on course to be reelected to a sixth term, in a scenario resembling a re-run of past presidential ballots.
As leader of the party with the most seats in parliament behind Deby's MPS, Felix Nialbe Romadoumngar is official head of the opposition and most likely to score significant support.
Meanwhile political newcomer Theophile Bongoro Bebzoune was briefly anointed the surprise replacement candidate of a coalition that had backed Kebzabo, before that alliance split apart.
Kebzabo and other opposition leaders have accused Bongoro of being "managed" by the Deby camp to divide the challengers.