The French were under arms long ere dawn, waiting for the signal to attack. The Portuguese also were awake and stirring in the darkness, when at three o’clock a thunderstorm, accompanied by a terrific hurricane from the north-west, swept over the city. In the midst of the elemental din some of the Portuguese sentinels thought that they had seen the French columns advancing to the assault: they fired, the artillery followed their example, and for half an hour the noise of the thunderstorm was rivalled by that of 200 guns of position firing at nothing. Just as the gunners had discovered their mistake, the tempest passed away, and soon after the day broke. So drenched and weary were the French, who had been lying down under the torrential rain, that Soult put off the assault for an hour, in order to allow them to dry themselves and take some refreshment; the pause also allowed the sodden ground to harden.
At seven all was again ready, and Merle’s and Delaborde’s regiments hurled themselves at the entrenchments above and below the city. Both made good progress, especially the former, who lodged themselves in the houses and gardens immediately under the main line of the Portuguese left wing, and captured several of its outlying defences. Seeing the position almost forced, Parreiras, the commander of the central part of the lines, acted just as Soult had hoped, and sent most of his reserve to reinforce the left. The Marshal then bade Merle halt for a moment, but ordered Delaborde, on his eastern flank, to push on as hard as he could. The general obeyed, and charged right into the Portuguese entrenchments, capturing several redoubts and actually breaking the line and getting a lodgement in the north-east corner of the city. Parreiras, to aid his colleague in this quarter, drew off many of his remaining troops, and sent them away to the right, thereby leaving his own section of the line only half manned. Thereupon Soult launched against the central redoubts his main assaulting column, Mermet’s division and the two regiments of dragoons. The central battalion went straight for the main position above the high-road, where the great Portuguese flag was flying on the strongest redoubt. The others attacked on each side. This assault was decisive: the Portuguese gunners had only time to deliver two ineffective salvos when the French were upon them. They charged into the redoubts through the embrasures, pulled down the connecting abattis, and swept away the depleted garrison in their first rush. The line of the defenders was hopelessly broken, and Mermet’s division hunted them down the streets leading to the river at full speed.