The officers would have been equipped with side arms and heavier weapons. The G36 was the traditional gun of choice but increasing numbers of officers are opting for SIG 516 weapons.
The first calls about a van hitting pedestrians on London Bridge would have triggered a major response and a set of challenges. The first aim would have been to find out what was going on, which can prove immensely difficult amid intense chaos and conflicting reports.
CCTV cameras belonging to police and others are plugged directly into Scotland Yard's control room and displayed on a phalanx of giant screens so commanders can see multiple scenes.
As officers raced towards the scene they would have been receiving information from the control room, based on what commanders could see, and reports from the members of the public. "Everything is based on the information coming to you en route," said one source. They would have been equipped with earpieces to continue the updates as they left their vehicles and started the search on foot for the suspects.
The Borough Market area is hugely popular for an evening out, and has narrow passages and windy streets, and a warren of pubs and restaurants.
Officers had to identify who was a reveller, scared after being caught up in an attack, and who was the attacker.
If time allowed, officers would have issued a shouted challenge to suspects, identifying themselves as armed police. The officers are trained and have previously been briefed to shoot to the head if necessary, known as a "critical shot". Armed officers are traditionally trained to shoot towards the centre of the chest, and pictures of the dead suspects suggest that happened in this case. "Without the luxury of time you would aim for the central body mass," said a source.
The firearms are equipped with laser sights, producing a so-called "red dot" on the suspect, showing the officer where the bullet will go.
"It's very accurate. It's point and shoot," said a police firearms expert.
Officers have to account for every shot fired and shoot to incapacitate the suspect, police chiefs say.
Normal post-shooting investigative steps were suspended and officers who had opened fire continued with colleagues with an "active search" for any other terrorist attackers. Usually they would have stopped their armed duties and started making statements on why the use of force was necessary.
The officers were beginning post-incident procedures on Sunday.
The increase in armed police numbers began after the 2015 attacks on Paris by marauding terrorist gunmen. British police chiefs got money to train more armed officers and developed new tactics.
In the Met area the number of ARV officers on duty at any time will increase by 50 per cent, and they train to mount armed operations with colleagues from the City of London force and British Transport police.
Informally Met chiefs believe armed officers can reach attacks in minutes. But among police there is concern about other areas of Britain, such as mid-size towns where armed officers have much greater geographic areas to cover. The concern is that the eight minutes seen on Saturday could be much longer.
"It was very lucky it was in an area with the highest number of armed officers around," said a source.
The improved police armed response is shown by the fact that it took 12 minutes to get 50 armed officers to the scene of the Westminster attack on 22 March, and 11 minutes to get armed officers to the scene of the Woolwich terror attack in 2013.
