Beefed-up security and road blocks set up late on Wednesday had been lifted in the area around the presidential palace, where traffic was back to normal, AFP journalists saw.

Heavy gunfire erupted near the presidential complex just before 8:00 pm local time (1900 GMT) on Wednesday in the centre of N'Djamena, the capital of the military-ruled, central African country.

Government spokesman Abderaman Koulamallah said a 24-member commando unit carrying "weapons, machetes and knives" attacked the guards of the presidential palace before being swiftly stopped.

The group was dressed in civilian clothing and came from a poor neighbourhood in the south of the city, he said.

Koulamallah described them as drunken "Pieds Nickeles" -- a reference to a French comic featuring hapless crooks.

He said late on Wednesday that 18 of the assailants were killed and six wounded. A presidential guard had also been killed and three others wounded.

Hours after the shooting, Koulamallah appeared in a video posted on Facebook, surrounded by soldiers and with a gun at his belt, saying: "The situation is completely under control... The destabilisation attempt was put down."

Koulamallah, who is also the foreign minister, told AFP that Chadian President Mahamat Idriss Deby Itno was inside the presidential palace at the time of the attack but gave no more details.

Opposition members have voiced doubts about the government's account of events.

Max Kemkoye, spokesman for the Political Actors' Consultation Group (GCAP), spoke on Thursday of an "unfortunate synopsis" and a "set up" orchestrated by those in power.

The government spokesman said he would make a statement to accredited diplomats later in the day and the prosecutor is also expected to make a statement.