Yaya Dillo Djerou was the leading opponent and a cousin of Mahamat Idriss Deby Itno, who was proclaimed transitional president by the country's ruling junta in 2021.
Dillo died on Wednesday after troops surrounded the office of his Socialist Party Without Borders in the capital N'Djamena, in what the party says was an "execution". The government denied the accusation.
Dillo, 49, had told AFP before his death that people wanted to "physically eliminate me" ahead of the election in which he -- and Deby Itno -- planned to run.
Accounts of what happened during the assault, when gunfire was heard, differ widely between government officials and the party.
"It's an execution, they fired at him at point-blank range to execute him for becoming a problem," his party's general secretary Robert Gamb said Friday.
Communications Minister Abderaman Koulamallah, the government's spokesman, rejected the accusation.
"We didn't execute anyone," he told AFP. "He opposed his arrest, there were exchanges of bullets.
"There was no execution."
The government denied the accusation, saying there was an exchange of gunfire when Dillo "opposed his arrest".
"The killing of a potential presidential candidate during an assault by Chadian security forces on an opposition party headquarters raises serious concerns about the environment for elections scheduled for May 6," HRW said in a statement.
"The circumstances of Yaya Dillo's killing are unclear, but his violent death highlights the dangers facing opposition politicians in Chad, particularly as elections approach," said Lewis Mudge, Central Africa director at HRW.
"The lack of clarity surrounding the attack on the PSF headquarters, the threats previously faced by Dillo, and the general political repression in the country all point to the need for an independent investigation with foreign assistance into the February 28, 2024 events," HRW said.
The rights organization added that it had "reviewed several photos sent by a reliable source close to Dillo, showing him dead with a single bullet wound to his head".
The violence came a day after Chad's military rulers announced a presidential election on May 6.
The voting would end a three-year transition period and aim to restore constitutional rule.
Dillo had told AFP before his death that people wanted to "physically eliminate me" ahead of the election in which he -- and Deby Itno -- planned to run.
The government had accused Dillo of leading an attack against the offices of Chad's internal security agency the night before his death. He denied any involvement.
In New York, UN Secretary-General Antonio Guterres's spokesman voiced concern about the killing.
"It's very important that there'd be restraint ahead of the first round of the presidential elections on May 6," Stephane Dujarric said.
"It's important that people respect the rule of law."
In Chad's capital, a large excavator was demolishing the three-story building housing the Socialist Party Without Borders on Friday afternoon.
An army security cordon kept people away and armored vehicles could be seen around the property.
'No weapon'
Security forces had this week sought the arrest of a member of Dillo's party over an alleged attempt to kill the Supreme Court president last month.
The government said the attack on the internal security offices, which killed several people, had been an act of reprisal.
Four soldiers and three supporters of Dillo died in Wednesday's fighting, according to the government.
Dillo had "retreated" to his party headquarters, Koulamallah said on Thursday, adding: "He didn't want to surrender and fired on law enforcement."
But his party and other opposition politicians have accused government forces of deliberately killing Dillo.
"You can't attack an opponent alone in an office with a whole arsenal of war," Gamb told AFP by telephone, adding that Dillo "had no weapon".
Dillo was a rebel who became a minister and then an opposition chief and regularly complained that the May ballot was being arranged to assure Deby Itno's victory.
The death of Dillo removes the transitional president's top rival in the vote.
"There isn't an opponent today who can pose a threat in the race for the presidency," said Enrica Picco of the International Crisis Group.
"With the assassination of Yaya Dillo, the government has struck a major blow before the elections so that everyone falls into line," said a regional African expert and diplomat, asking to remain anonymous.
"It's a very strong message sent to the opposition, to its own skeptics, and the Zaghawas," he said. The transitional president "wants to short-circuit any opposition".
Dillo and his cousin are from the same Zaghawa ethnic minority, which has dominated Chad's politics for more than three decades.
The nation is part of central Africa's tumultuous Sahel region that has been gripped by jihadist insurgencies for more than a decade.
Chad is the last Sahel nation to house French troops following the forced withdrawal of French soldiers from Mali, Burkina Faso, and Niger in the last couple of years following coups.
Paris has remained silent over recent events in N'Djamena after strongly criticizing military rule in Mali, Burkina, and Niger.
"There is a risk of (France) being accused of double standards," said Elie Tenenbaum of the French Institute for International Relations (IFRI). "It can be politically embarrassing."
Deby Itno took power after the death of his father, veteran leader Idriss Deby Itno, in 2021 while fighting rebels.
He promised a return to civilian rule and elections within 18 months but extended the transition by two years.
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