With hundreds of thousands of people expected to line the streets of London for a ceremonial procession on Wednesday, a former U.K. counter-terrorism co-ordinator warned that the security challenges around the Queen's funeral are "enormous."

In addition to the large crowds gathered today for the ceremonial procession to move Queen Elizabeth's coffin from Buckingham Palace to Westminster Hall — where the former monarch will lie in state until Sept. 19 — officials are dealing with the logistics of holding a funeral that will be attended by hundreds of world leaders and dignitaries. 

I think that is a different quantum of risk," said Nick Aldworth, who held many top jobs during his three-decade career as a public security official, including head of security and policing at the British Parliament, managing security for the Queen Mother and firearms co-ordinator for the 2012 London Olympics.