From: http://militaryhistory.about.com/od/americanrevolution/p/lexconcord.htm
Old North Bridge Battle and subsequent British March:
Seeing the smoke from the fire, Barrett and his men moved closer to the bridge and saw around 90-95 British troops fall back across the river. Advancing with 400 men, they were engaged by the British. Firing across the river, Barrett's men forced them to flee back towards Concord. Unwilling to initiate further action, Barrett held his men back as Smith consolidated his forces for the march back to Boston. After a brief lunch, Smith ordered his troops to move out around noon. Throughout the morning, word of the fighting had spread, and colonial militias began racing to the area.

Aware that his situation was deteriorating, Smith deployed flankers around his column to protect against colonial attacks as they marched. About a mile from Concord, the first in a series of militia attacks began at Meriam's Corner. This was followed by another at Brooks Hill. After passing through Lincoln, Smith's troops were attacked at the "Bloody Angle" by 200 men from Bedford and Lincoln. Firing from behind tree and fences, they were joined by other militiamen who took up positions across the road, catching the British in a crossfire.