Complex sentence: Three days before the event, organisers realised the march would be larger than 60,000 (the capacity of Trafalgar Square) and asked permission from the MPS and the Department of the Environment to divert the march to Hyde Park.
Split into simpler sentences: Three days before the event the organisers realised the march would be larger than the 60,000 capacity of Trafalgar Square. It asked permission from the Metropolitan Police Service and the Department of the Environment to divert the march to Hyde Park.