Attempts by militants to detonate more bombs at last Thursday's terror attacks in Central Jakarta were foiled by an off-duty police officer who shot and killed one attacker, while incapacitating another - both at close range.

The officer, Lieutenant-Colonel Untung Sangaji, told reporters yesterday he had no choice but to gun down Sunakim alias Afif, the most recognisable of the four militants who mounted the attack.

This was because he saw that Sunakim was about to hit a switch that was likely to detonate a bomb he was carrying in his backpack.

The instructor at the Maritime Police Training Centre also shot at the legs of a second militant, possibly Muhammad Ali, whom Lt-Col Untung believed also had a detonator. Like Sunakim, Ali had a pistol in his right hand. Both men had been shooting randomly into the crowds and at police officers.

An Explosive Ordnance Disposal officer later recovered from Ali's bag a homemade bomb that was 40cm long and 15cm in diameter.

"If this had exploded, the impact and shrapnel would probably be as powerful as 10 grenades," said Lt-Col Untung, who was previously with the police bomb squad.

A total of six unexploded bombs - one from Ali and five smaller ones - were found at the scene after all four militants were killed, the police had said previously.

Lt-Col Untung was recounting his actions - which were also captured on closed-circuit TV - during a press conference yesterday at the national police headquarters.

He said he was near Jalan M. H. Thamrin having coffee with a colleague minutes before the attack. They had left for a cigarette when the first explosions went off outside the Starbucks cafe in the Menara Cakrawala building.

"I rushed over and saw the police post at the nearby traffic junction in smoke," he said. "When I got to the police post, I heard someone from inside shouting: 'Help, help, a police officer is injured.' "

He added that a crowd had already gathered at the scene - not to help, but to take photos with their cellphones. "Seconds later, there was a sound, 'bang-bang', and one of those people who were taking a selfie fell over."

Lt-Col Untung immediately shouted for everyone to "get down", drew his pistol and went after the shooter with a fellow officer he referred to only as Tamat.

"I spotted the shooter moving in one direction - he was not running, just walking," he said. "He was carrying a backpack and one of his hands held up the bottom of the backpack, probably because it was heavy."

During the exchange of gunfire with the officers, Sunakim and Ali took cover beside a car in a carpark outside Starbucks.

Lt-Col Untung told Tamat to get down on the ground and aim for the militants' legs while he shot through the car's windows.

Sunakim, 34, and Ali, 39, had recently been released from prison after serving time for terror-related offences.

Police earlier identified their accomplices as Dian Joni Kurniadi and Ahmad Muhazin, both 25.

The police believed that they had been acting under the direction of Bahrun Naim, an Indonesian militant loyal to the Islamic State in Iraq and Syria.

So far 12 other suspects, believed to have ties to Bahrun's militant cell in Indonesia, have been rounded up since Thursday.

Correction note: An earlier version of this report stated that the militant Sunakim alias Afif's age was 32. A source, who asked not to be named, has since clarified that he was 34.