Heavy gunfire has been heard in Chad's capital N'Djamena following a deadly attack on the headquarters of the National Security Agency (ANSE).

Several people were killed in Wednesday's attack, the government said - blaming the opposition Socialist Party Without Borders (PSF).

Its leader, Yaya Dillo, told the AFP news agency the allegation was a "lie".

The unrest comes a day after the announcement that Chad will hold presidential elections on 6 May.

N'Djamena residents reported hearing intense gunfire near the PSF's main office on Wednesday and said they had seen several military vehicles heading there.

One witness told Reuters that the PSF buildings had been cordoned off.

Communication Minister Abderaman Koulamallah said the earlier attack on the ANSE buildings was led by Mr Dillo, who denied he was there - telling AFP the accusation was intended "to make me afraid so that I don't go to the election".

One of Mr Dillo's colleagues, the PSF's general secretary, told Reuters that contrary to the government's claim that its members had attacked the ANSE building, they had been the ones to come under attack from soldiers while trying to retrieve the body of their colleague Ahmed Torabi.

The PSF official said Mr Torabi was arrested and shot dead on Tuesday, before his body was dumped outside the ANSE buildings.

According to the government, Mr Torabi had attempted to assassinate the president of the Supreme Court.

Relatives and party members who tried to get to his body were shot at on Wednesday morning, resulting in multiple fatalities, the general secretary said.

Mr Dillo also denied any links to the assassination attempt, which he described as "staged", according to AFP.

The government said PSF members had been arrested or were being sought over the attack on the ANSE headquarters and would be prosecuted.

"Anyone looking to disturb the democratic process under way in the country will be prosecuted and brought to justice," the government said in a statement quoted by AFP.

It is not clear if Mr Dillo was among those arrested, but in a Facebook post on Wednesday morning he said the military had come for him at his party headquarters.

In addition to the violence, internet connectivity has been disrupted in the country, according to internet watchdog Netblocks.

Mr Dillo is a vocal opponent of his cousin, President Mahamat Déby, who came into power in 2021 after the latter's father was killed by rebels after three decades in power.

Mr Déby promised to return the country to civilian rule - but delayed it for more than two years.

The May election is supposed to mark the end of the political transition.

The Patriotic Salvation Movement (MPS) designated President Déby as its candidate for the upcoming elections, but he is yet to openly comment on whether he will run.

Former colonial power France has been backing Mr Déby since the start of the transition, raising eyebrows both in and out of the country.

France currently has about 1,000 troops in Chad to fight jihadist groups across West Africa.

The opposition said the electoral commission is far from neutral and it fears an extension of the Déby dynasty.