First Battle of Porto In the First Battle of Porto ( March 28 , 1809 ) the French under Marshal Soult defeated the Portuguese , under General Parreiras , and the British Army , outside the city of Porto during the Peninsular War . Soult followed up his success by storming the city . It is estimated that 8 , 000 soldiers perished in the attack and that a great number of civilians were killed . Soult ' s invasion of Portugal . After the Battle of Corunna , Napoleon ordered Marshal Nicolas Soult to invade Portugal from the north . He was to seize Porto by 1 February and Lisbon by 10 February . Napoleon failed to take into account either the wretched condition of the roads or the fact that a full - scale guerrilla war had broken out in Northern Portugal and Spain . Soult ' s II Corps had four infantry divisions , commanded by Generals of Division Pierre Hugues Victoire Merle , Julien Augustin Joseph Mermet , Étienne Heudelet de Bierre , and Henri François Delaborde . Merle had four battalions each of the 2nd Light , 4th Light and 15th Line Infantry Regiments and three battalions of the 36th Line . Mermet ' s division included four battalions each of the 31st Light , 47th Line , and 122nd Line , and one battalion each of the 3rd , 4th and 5th Swiss Regiments . Heudelet led two battalions each of the 22nd Line and 66th Line , one battalion each of the 15th Light , 32nd Light , 82nd Line , " Légion du Midi " , Paris Guard , and Hanoverian Legion . Delaborde ' s command comprised three battalions each of the 17th Light , 70th Line , and 86th Line . General of Division Jean Baptiste Marie Franceschi - Delonne led Soult ' s corps cavalry , the 1st Hussar , 8th Dragoon , 22nd Chasseur à Cheval , and Hanoverian Chasseur Regiments . Attached were General of Division Armand Lebrun de La Houssaye ' s 3rd Dragoon Division and General of Division Jean Thomas Guillaume Lorge ' s 4th Dragoon Division . The 3rd Dragoon Division was made up of the 17th , 18th , 19th and 27th Dragoon Regiments . The 4th Dragoon Division consisted of the 13th , 15th , 22nd and 25th Dragoon Regiments . In all , Soult had 23 , 500 men , including 3 , 100 cavalry . Soult ' s first attempt to invade Portugal was stopped by the local militia on February 16 . The French then moved northeast to Orense in Spain , seized an unguarded bridge and marched south . On the way , Franceschi ' s cavalry overran Major General Nicolas Mahy ' s Spanish brigade at La Trepa on March 6 , inflicting 700 casualties . The French crossed into Portugal and occupied Chaves on March 9 . From Chaves , Soult moved west to Póvoa de Lanhoso where he was confronted by Baron Christian Adolph Friedrich von Eben ' s 25 , 000 - man army composed mostly of Portuguese militia armed with muskets , pikes , and agricultural implements . After waiting several days for all his troops to arrive , Soult went over to the attack . On 20 March 1809 in the Battle of Braga the French veterans butchered their adversaries . The outmatched Portuguese lost 4 , 000 killed and 400 captured . The French , who lost 40 killed and 160 wounded , also seized 17 Portuguese cannons . The battle . Bishop Castro organized an army of 24 , 000 men to defend Porto . Generals Lima and Parreiras commanded two battalions , each of the 6th , 18th and 21st Infantry Regiments , and one battalion of the 9th and other units . The 4 , 500 Portuguese regulars were supported by 10 , 000 " ordenanças " ( militia ) and 9 , 000 armed citizens . When Soult hurled Merle , Mermet , Heudelet , Franceschi and Lahoussaye at the Portuguese deployed north of the city , on the weakest part of the Portuguese line of defence , Castro ' s force soon dissolved and the battle became a massacre . The Portuguese tried to escape from the French in the city but were chased by the French cavalry throughout the streets , and their regular units were annihilated . Thousands of fleeing civilians drowned when a bridge of boats across the Douro River ( as soon as some Portuguese units started to sabotage the bridge to prevent the French from crossing the river ) collapsed because of their weight and of Portuguese artillery fire ( coming from the left side of the Douro ) who were aiming at the French cavalry behind the Portuguese soldiers and citizens . In the roadstead , Soult captured a squadron of Spanish naval vessels and 30 merchant ships . The French also found large stockpiles of British military stores . In the battle and storming of the city , the French lost 72 officers and 2 , 000 rank and file casualties . The Portuguese lost about 8 , 000 killed and 197 cannons captured . Soult did not have very long to enjoy his success . Almost at once , the " ordenanças " cut his communications with Spain and a 1 , 800 - man garrison was gobbled up by Francisco Silveira ' s Portuguese force in the Siege of Chaves . The French marshal started planning a retreat . The next action was the Battle of Grijó . The city was retaken on May 12 by the British and Portuguese under Wellesley in the Second Battle of Porto .