Guard Collapses While Standing Watch at Queen Elizabeth II's Coffin

One of the Royal guards took a rough fall at Westminster Hall ... smacking HARD onto the floor as he stood watch beside Queen Elizabeth II's coffin.

The guard was swaying side to side at the foot of the casket Wednesday before landing face-first on the ground ... the crash echoed through the Hall, with people rushing to his aid.

Royal guards normally rotate around the coffin every 20 minutes, with a full shift lasting 6 hours -- The BBC, which was streaming the ceremony, cut the feed until the incident was resolved.

It's unclear what caused the guard to collapse. No word on his condition either.

The guard wasn't the only one losing consciousness at Westminster Hall ... 41-year-old Lady Gabriella Windsor, the cousin of Queen Elizabeth II, collapsed earlier in the service.

Lady Gabriella was in the receiving line with other members of the Royals when several people began to look concerned ... including her mother, Princess Michael of Kent.

Her husband, Thomas Kingston, rushed to her aid. She reportedly regained consciousness but left the event.