All three men strapped on the fake belts – which were actually water bottles covered in masking tape – as they carried out the assault.
Counter-terror Commander Dean Haydon said: “I’ve not seen this tactic in the UK before where terrorists create maximum fear with fake explosives. It could be that they had plans to take the attack into a siege situation.
“Or it might be that they saw it as protection from being shot themselves. If you are fighting back you would be very aware you could be caught in an explosion.
Inspector Cole added: “Without a shadow of a doubt, it was the most challenging, most intense situation I’ve dealt with.
“I’ve dealt with a lot of death and I’ve been to some pretty horrific scenes in my career, but nothing has ever been on that scale. It’s going to stick with me for a long time.”
The twisted murderers killed eight people in the rampage, which began when they ploughed a van into pedestrians on London Bridge before they moved towards Borough Market, where they began stabbing people indiscriminately.
The three terrorists were shot within eight minutes of the first call to emergency services.
Six men remain in police custody as part of the investigation into the attack.
Twelve people were arrested the day after the attack took place, but all were released without charge.