On April 19, 1775, British regulars marched from Boston to seize colonial military supplies at Concord. Warned by riders including Paul Revere, militia assembled at Lexington, where shots fired at dawn killed eight colonists. At Concord's North Bridge, over 300 militia forced the British covering party to withdraw. The 20-mile return march became an ordeal as colonists fired from behind walls, trees, and buildings. British losses totaled 273; American losses 95. Within days, 16,000 New Englanders mobilized and began the Siege of Boston.