They ran in mud, treaded knee-deep in mud, even swam in muddy waters.

Thousands of athletes turned out at a military base near Madrid in sweltering heat on Saturday to compete in a very dirty obstacle course event.

The Mud Day race was held under the slogan "running is not enough" and drew many people eager to navigate a slippery course that included tunnels, dunes and even barbed electric wire.

Created in 2013, the Mud Day race has held previous competitions in France. Upcoming stops in Spain include Valencia, Seville and Barcelona.