Four people were injured, three seriously, in the incident on 7 March 2012.
The hoarding, which was some 3.6m high and weighed nearly a tonne, had been put up the previous day by Wiltshire-based Oracle Interiors Ltd to fence off a clothing store that was being refurbished.
The Health & Safety Executive (HSE) prosecuted after identifying serious flaws with the temporary structure.
Westminster Magistrates’ Court heard that the hoarding was held in place by just a single timber brace.
An estimated 20 people were trapped by the hoarding when it came down, although most managed to escape unharmed as emergency crews and fellow passers-by rushed to help. Among the injuries were several broken bones.
Oracle Interiors Ltd, of Lysander Way, Salisbury, was fined £10,000 and ordered to pay £13,069 in costs after pleading guilty to a single breach of the Construction (Design and Management) Regulations 2007.
HSE Inspector Wendy Garnett said after the hearing: “The law clearly states that all temporary structures, including hoardings, should be properly designed, and so installed as to withstand any foreseeable loads imposed on them. That clearly wasn’t the case on this occasion and innocent shoppers were subjected to a frightening and, for some, hugely traumatic ordeal that had a long-term impact.”