The startling incident happened as guards were between shifts, and the guard shook before falling.

Credits: Getty Images

The video ends with two police officers rushing to grab the security guard before it stops recording.

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Guards of honour defending Queen Elizabeth In a sad procession through the flag-draped, crowd-lined streets of London to Westminster Hall on Wednesday.

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In a sad procession through the flag-draped, crowd-lined streets of London to Westminster Hall on Wednesday.

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There, the world may grieve for the longest-reigning monarch of Great Britain.

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The scenario couldn't have been more theatrical when the guard suddenly became ill

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The Westminster Hall was packed with troops and mourners paying their final respects to the Queen.

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The space filled with echoes of gasps and dread.

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