The sound of automatic weapon fire prompted people to leave the area in the center of N'Djamena where the Socialist Party Without Borders (PSF) has its main office and had been surrounded by the army.
Presidential guard trucks were also seen headed towards the party headquarters, an AFP journalist saw.
Since around midday, the telephone network and internet were disrupted, according to AFP journalists.
A PSF official requesting anonymity told AFP by telephone of Kalashnikov assault rifles and teargas grenades near the party headquarters.
The attack on the security offices came after a PSF member was arrested and accused of an "assassination attempt against the president of the supreme court," a government statement said.
It said the situation appeared to have "taken a dramatic turn" with "a deliberate attack by accomplices of this individual led by elements of the PSF and at their head, the president of this movement Yaya Dillo" against the state security offices.
"The situation is now completely under control," the government said.
"The perpetrators of this act have been arrested or are being sought and will be prosecuted."
Dillo, who heads the PSF party and is a fierce opponent of Chad's transitional president, his cousin Mahamat Idriss Deby Itno, denied any involvement in the incident.
Speaking to AFP, he denounced the claim as a "lie," saying: "I wasn't present."
"The desired goal is to prevent me, to physically eliminate me... to make me afraid so that I don't go to the election," Dillo said.
He earlier had condemned the allegations of an attempted attack against the supreme court president as "staged."
Brought to justice
The attack on the security offices comes a day after Chad announced it would hold a presidential election on May 6, which both Deby Itno and Dillo intend to contest.
"Anyone looking to disturb the democratic process under way in the country will be prosecuted and brought to justice," the government said Wednesday.
Deby Itno took power in Chad at the age of 37 when the army proclaimed him transitional president on the death of his father Idriss Deby Itno, who was killed while fighting rebels in 2021.
The elder Deby Itno had come to power in a 1990 coup and ruled the desert nation with an iron fist for three decades.
Mahamat Deby Itno promised to hand power back to civilians and organize elections within 18 months, but subsequently added another two years of transition.
The end of the transition period was pushed back to October 10 this year.
The Chadian opposition has asked the transitional president not to run for office in the central African country, which is the second least developed in the world, according to the United Nations.
The leading opposition and civil society grouping Wakit Tamma has accused the international community, and former colonial ruler France in particular, of supporting "dynastic succession."
Yaya Dillo was a candidate for the presidency in 2021 against his uncle, Idriss Deby Itno.
Dillo fled the country in February of that year after security forces attempted to arrest him at his home.
The commando-style raid left several dead including his mother and one of his sons.
He was accused by the regime of Idriss Deby Itno of making corruption accusations against the first lady.
In mid-January, the ruling Patriotic Salvation Movement (MPS) party designated the transitional leader as its candidate for the presidential election.
He had told the African Union he would not run, but a new constitution adopted in a December referendum allows him to do so.
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