Another shopper, Anna Morrell, told the BBC: "Myself and my child were just walking across Regent Street and a metal diamond knife fell from the sky.
"I think it's incredibly dangerous. It will slice somebody if they're walking underneath that."
Crown Estate, which oversees the decorations, said "a number" of the shapes came loose in strong winds.
A spokesman added: "We place the highest priority on health and safety and our specialist installation team are on-site and continuing to keep the situation under review, including undertaking testing and reinforcement of the decorations.”
