At nine o'clock in the evening it became known that the general plan of attack predicted by Major Harrison was to be carried out. Some five thousand men under General Ferguson were to ascend the hills on the left of the valley, while Trant, with a thousand Portuguese infantry and some Portuguese horse, were to move on the hills on the right; the centre, nine thousand strong, and commanded by Sir Arthur himself, were to march straight up the valley.

Early in the morning the British troops marched out from Obidos. Ferguson's command at once turned to the left and ascended the hills, while Trant's moved to the west.

After proceeding a short distance, Fane's brigade moved off from the road and marched along the valley, equidistant from the main body and from Ferguson, forming a connecting link between them; and on reaching the village of St. Mamed, three-quarters of a mile from the French position, Hill's brigade turned off to the right. From their elevated position the French opened fire with their artillery, and this was answered by the twelve guns in the valley and from Ferguson's six guns on the heights. Fane's brigade, extended to its left, was the first in action, and drove back the French skirmishers and connected Ferguson with the centre. They then turned to attack the right of the French position; while Ferguson, seeing no signs of Loison's force, descended from the high ground to the rear of Fane, while the Portuguese pressed forward at the foot of the hills on the other side of the valley and threatened the enemy's left flank.

Seeing that his position was absolutely untenable, Laborde did not wait the assault, but fell back, covered by his cavalry, to the far stronger position in his rear. A momentary pause ensued before the British continued their advance. The new position of the French was of great natural strength, and could be approached only by narrow paths winding up through deep ravines on its face. Ferguson and Fane received orders to keep to the left, and so turn the enemy's right. Trant similarly was to push forward and threaten his left flank, while Hill and Nightingale advanced against the front.

The battle commenced by a storm of skirmishers from these brigades running forward. These soon reached the foot of the precipitous hill and plunged into the passes. Neither the fire of the enemy nor the difficulties of the ascent checked them. Spreading right and left from the paths they made their way up, and taking advantage of the shelter afforded by great boulders, broken masses of rock, and the stumps of trees, climbed up wherever they could find a foothold. The supporting columns experienced much greater difficulty; the paths were too narrow, and the ground too broken for them to retain their formation, and they made their way forward as best they could in necessary disorder.

The din of battle was prodigious, for the rattle of musketry was echoed and re-echoed from the rocks. The progress of the skirmishers could only be noted by the light smoke rising through the foliage and by the shouts of the soldiers, which were echoed by the still louder ones of the French, gathered strongly on the hill above them. As the British made their way up, Laborde, who was still anxiously looking for the expected coming of Loison, withdrew a portion of his troops from the left and strengthened his right, in order to hold on as long as possible on the side from which aid was expected. The ardour of the British to get to close quarters favoured this movement.

It had been intended that the 9th and 29th Regiments should take the right-hand path where the track they were following up the pass forked, and so join Trant's Portuguese at the top of the hill and fall upon the French left. The left-hand path, however, was the one that would take them direct to the enemy, and the 29th, which was leading, took this, and the 9th followed them. So rapidly did they press up the hill that they arrived at the crest before Ferguson and Fane, on the left, and Trant on the right, had got far enough to menace the line of retreat, and so shake the enemy's position. The consequence was, that as the right wing of the 29th arrived at the top of the path it was met by a very heavy fire before it could form, and some companies of a French regiment, who had been cut off from the main body by its sudden appearance, charged through the disordered troops and carried with them a major and fifty or sixty other prisoners.

The rest of the wing, thus exposed to the full fire of the French, fell back over the crest, and there rallied on the left wing; and being joined by the 9th, pushed forward again and obtained a footing on the plateau. Laborde in vain endeavoured to hurl them back again. They maintained their footing, but suffered heavily, both the colonels being killed, with many officers and men. But the 5th Regiment were now up, and at other points the British were gathering thickly at the edge of the plateau. Ferguson and Trant were pushing on fast past the French flanks, and Laborde, seeing that further resistance would lead to great disaster, gave the order to retire to a third position, still farther in the rear. The movement was conducted in splendid order. The French steadily fell back by alternate masses, their guns thundering on their flanks, while their cavalry covered the rear by repeated charges.

Gaining the third position, Laborde held it for a time, and so enabled isolated bodies of his force to join him. Then, finding himself unable to resist the impetuosity of the British attack, he retired, still disputing every foot of ground, and took to the narrow pass of Runa. He then marched all night to the strong position of Montechique, thereby securing his junction with Loison, but leaving the road to Torres Vedras open to the British. The loss of the French in this fight was 600 killed and wounded, and three guns. Laborde himself was among the wounded. The British lost nearly 500 killed, wounded, or taken prisoners. The number of the combatants actually engaged on either side was about 4,000, and the loss sustained showed the obstinacy of the fighting. Sir Arthur believed that the French had, as they retreated, been joined by Loison, and therefore prepared to march at once by the coast-line to seize the heights of Torres Vedras before the French could throw themselves in his way.