London Bridge isn't falling down -- but this London police officer definitely is after whacking her head into a street light ... leaving her trusty steed riderless.

This wild scene unfolded Saturday from across the pond, where people were putting on BLM protests decrying the death of George Floyd. Scotland Yard had their hands full managing things, so at one point ... they trotted out their boys and girls on horses to establish order.

That didn't go so well for one cop whose equine went full bronco and rode off into the night day ... galloping down the street ahead of all the others, leaving the officer face to metal with a traffic light fixture dangling above.

The female officer was knocked completely off the animal and landed hard on the ground, with the crowd in shock. She had a helmet on, but still ... ouch. Photos of the woman on the ground were also taken by those on the scene -- reports say she was knocked unconscious and taken to a hospital, where she was treated for non-life-threatening injuries.

Unclear what prompted the horse to freak out and jolt like that, but based on other video that showed protesters rolling bikes into them from a distance ... it's reasonable to assume it got spooked with all the chaos around it.

For a quick minute, this horse was on the loose and on the run, and it even appears it might've ran into a bystander as it ran down the street. People say this lady was trampled, but it looks like she was simply knocked over. Either way, super frightening situation.