Chad's main opposition party on Friday accused soldiers of having executed its leader "at point blank range" in an assault on its headquarters ahead of a long-promised May election.

Yaya Dillo Djerou was the leading opponent and cousin of Mahamat Idriss Deby Itno, who was proclaimed transitional president by the 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.

The violence came a day after Chad's military rulers announced a presidential election on May 6.

The polls will end a three-year transition period and aim to return the central African country to constitutional rule.

Dillo, 49, had told AFP before his death that people wanted to "physically eliminate me" ahead of the election, which he -- and Deby Itno -- planned to contest.

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 charged Friday.

Communications Minister Abderaman Koulamallah immediately refuted the accusation.

"We didn't execute anyone," he told AFP by telephone. "He opposed his arrest, there were exchanges of bullets.

"There was no execution," Koulamallah, who is also the government spokesman, said.

The capital was calm Friday but a large excavator was demolishing the three-storey building housing the Socialist Party Without Borders on Friday afternoon, AFP journalists saw.

An army security cordon kept people well back and armoured vehicles could be seen around the property.

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.

- '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" and "who had no weapon", Gamb told AFP by telephone.

Dillo was a rebel who became a minister and then an opposition chief and regularly complained that the May ballot was arranged to assure a victory for Deby Itno.

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," noted Enrica Picco of the International Crisis Group.

The two men were from the same Zaghawa ethnic minority, which for more than three decades has dominated Chad's politics.

Deby Itno seized 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.