The belts, created to look like real explosives, consisted of three disposable water bottles covered in masking tape that were then attached to the belt.
United Kingdom police Commander Dean Haydon, who is leading the investigation, said: "I have not seen this tactic in the United Kingdom before where terrorists create maximum fear by strapping fake explosives to themselves".
The 34-year-old said: "They were behind me so I couldn't see, one was standing basically beside me as I was huddled down in the corner".
The attackers, identified as Khuram Butt, Rachid Redouane and Youssef Zaghba, were fatally shot by police.
Roses with messages are given out to passers-by on London Bridge just over a week on from the terror attack on the bridge and Borough Market, Sunday June 11, 2017.
Three 12-inch pink ceramic kitchen knives were found with tape around the handles and leather straps attached to them to keep the weapons on the attackers' wrists. It was the third such deadly attack in Britain in three months.
Police arrested 20 people after the attack and seven remained in custody on Sunday.
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They say the attackers may have planned even worse carnage.
Officers from the Metropolitan Police's Counter Terrorism Command arrested a 28-year-old man at a residential address in East London at 2:05 a.m., police said.
Metropolitan Police Commander Dean Haydon praised the bravery of the police officers and members of public who tackled the three despite the danger of being killed in an explosion had the vests been real.
Police also found the attackers had hired a flat at Barking, south London, as the safe house for them to prepare the attack, in which petrol bombs have been found.
Twelve other people who were detained in the wake of the van and knife attack in London were released without charge.
Police also revealed Butt had attempted to hire a 7.5-tonne lorry online, but failed to provide payment details.
They killed eight people before they were brought down by London cops. "I got them into the pub's basement as a place of hard cover", he said.