Early on Monday morning, 24 April 1916, roughly 1,200 Volunteers and Citizen Army members took over strongpoints in Dublin city centre. A joint force of about 400 Volunteers and Citizen Army gathered at Liberty Hall under the command of Commandant James Connolly.
The rebel headquarters was the General Post Office (GPO) where James Connolly, overall military commander and four other members of the Military Council: Patrick Pearse, Tom Clarke, Seán Mac Dermott and Joseph Plunkett were. After occupying the Post Office, the Volunteers hoisted two republican flags and Pearse read a Proclamation of the Republic.
Elsewhere, rebel forces took up positions at the Four Courts, the centre of the Irish legal establishment, at Jacob's Biscuit Factory, Boland's Mill, the South Dublin Union hospital complex and the adjoining Distillery at Marrowbone Lane. Another contingent, under Michal Mallin, dug in on St. Stephen's Green.
Although it was lightly guarded, Volunteer and Citizen Army forces under Seán Connolly failed to take Dublin Castle, the centre of British rule in Ireland, shooting dead a police sentry and overpowering the soldiers in the guardroom, but failing to press home the attack. The Under-Secretary for Ireland, Sir Matthew Nathan, alerted by the shots, helped close the castle gates. The rebels occupied the Dublin City Hall and adjacent buildings. They also failed to take Trinity College, in the heart of the city centre and defended by only a handful of armed unionist students. At midday a small team of Volunteers and Fianna Éireann members attacked the Magazine Fort in the Phoenix Park and disarmed the guards, with the intent to seize weapons and blow up the building as a signal that the rising had begun. They set explosives but failed to obtain any arms.
In at least two incidents, at Jacob's and Stephen's Green, the Volunteers and Citizen Army shot dead civilians trying to attack them or dismantle their barricades. Elsewhere, they hit civilians with their rifle butts to drive them off. The British military were caught totally unprepared by the rebellion and their response of the first day was generally un-coordinated. Two troops of British cavalry, one at the Four Courts and the other on O'Connell Street, sent to investigate what was happening took fire and casualties from rebel forces. On Mount Street, a group of Volunteer Training Corps men stumbled upon the rebel position and four were killed before they reached Beggars Bush barracks.
The only substantial combat of the first day of the Rising took place at the South Dublin Union where a piquet from the Royal Irish Regiment encountered an outpost of Éamonn Ceannt's force at the northwestern corner of the South Dublin Union. The British troops, after taking some casualties, managed to regroup and launch several assaults on the position before they forced their way inside and the small rebel force in the tin huts at the eastern end of the Union surrendered. However, the Union complex as a whole remained in rebel hands. Three unarmed Dublin Metropolitan Police were shot dead on the first day of the Rising and their Commissioner pulled them off the streets. Partly as a result of the police withdrawal, a wave of looting broke out in the city centre, especially in the O'Connell Street area. A total of 425 people were arrested after the Rising for looting.
A British armoured truck, hastily built from the smokeboxes of several steam locomotives at Inchicore railway works Lord Wimborne, the Lord Lieutenant, declared martial law on Tuesday evening and handed over civil power to Brigadier-General William Lowe. British forces initially put their efforts into securing the approaches to Dublin Castle and isolating the rebel headquarters, which they believed was in Liberty Hall. The British commander, Lowe, worked slowly, unsure of the size of the force he was up against, and with only 1,269 troops in the city when he arrived from the Curragh Camp in the early hours of Tuesday 25 April. City Hall was taken from the rebel unit that had attacked Dublin Castle on Tuesday morning.
The rebels had failed to take either of Dublin's two main train stations or either of its ports, at Dublin Port and Kingstown. As a result, during the following week, the British were able to bring in thousands of reinforcements from England and from their garrisons at the Curragh and Belfast. By the end of the week, British strength stood at over 16,000 men. Their firepower was provided by field artillery summoned from their garrison at Athlone which they positioned on the northside of the city at Phibsborough and at Trinity College, and by the patrol vessel Helga, which sailed up the River Liffey, having been summoned from the port at Kingstown. On Wednesday, 26 April, the guns at Trinity College and Helga shelled Liberty Hall, and the Trinity College guns then began firing at rebel positions, first at Boland's Mill and then in O'Connell Street.
The principal rebel positions at the GPO, the Four Courts, Jacob's Factory and Boland's Mill saw little combat. The British surrounded and bombarded them rather than assault them directly. One Volunteer in the GPO recalled, 'we did practically no shooting as there was no target'. Similarly, the rebel position at St Stephen's Green, held by the Citizen Army under Michael Mallin, was made untenable after the British placed snipers and machine guns in the Shelbourne Hotel and surrounding buildings. As a result, Mallin's men retreated to the Royal College of Surgeons building where they remained for the rest of the week. However, where the insurgents dominated the routes by which the British tried to funnel reinforcements into the city, there was fierce fighting.
Reinforcements were sent to Dublin from England, and disembarked at Kingstown on the morning of 26 April. Heavy fighting occurred at the rebel-held positions around the Grand Canal as these troops advanced towards Dublin. The Sherwood Foresters were repeatedly caught in a cross-fire trying to cross the canal at Mount Street. Seventeen Volunteers were able to severely disrupt the British advance, killing or wounding 240 men. Despite there being alternative routes across the canal nearby, General Lowe ordered repeated frontal assaults on the Mount Street position. The British eventually took the position, which had not been reinforced by the nearby rebel garrison at Boland's Mills, on Thursday but the fighting there inflicted up to two thirds of their casualties for the entire week for a cost of just four dead Volunteers.
The rebel position at the South Dublin Union (site of the present day St. James's Hospital) and Marrowbone Lane, further west along the canal, also inflicted heavy losses on British troops. The South Dublin Union was a large complex of buildings and there was vicious fighting around and inside the buildings. Cathal Brugha, a rebel officer, distinguished himself in this action and was badly wounded. By the end of the week, the British had taken some of the buildings in the Union, but others remained in rebel hands. British troops also took casualties in unsuccessful frontal assaults on the Marrowbone Lane Distillery.
The third major scene of combat during the week was at North King Street, behind the Four Courts, where the British, on Thursday, tried to take a well-barricaded rebel position. By the time of the rebel headquarters' surrender, the South Staffordshire Regiment under Colonel Taylor had advanced only 150 yd (140 m) down the street at a cost of 11 dead and 28 wounded. The enraged troops broke into the houses along the street and shot or bayoneted 15 male civilians whom they accused of being rebel fighters.
Elsewhere, at Portobello Barracks, an officer named Bowen Colthurst summarily executed six civilians, including the pacifist nationalist activist, Francis Sheehy-Skeffington. These instances of British troops killing Irish civilians would later be highly controversial in Ireland.
The headquarters garrison at the GPO, after days of shelling, was forced to abandon their headquarters when fire caused by the shells spread to the GPO. Connolly had been incapacitated by a bullet wound to the ankle and had passed command on to Pearse. The O'Rahilly was killed in a sortie from the GPO. They tunnelled through the walls of the neighbouring buildings in order to evacuate the Post Office without coming under fire and took up a new position in 16 Moore Street. On Saturday 29 April, from this new headquarters, after realising that they could not break out of this position without further loss of civilian life, Pearse issued an order for all companies to surrender. Pearse surrendered unconditionally to Brigadier-General Lowe. The surrender document read:
In order to prevent the further slaughter of Dublin citizens, and in the hope of saving the lives of our followers now surrounded and hopelessly outnumbered, the members of the Provisional Government present at headquarters have agreed to an unconditional surrender, and the commandants of the various districts in the City and County will order their commands to lay down arms.
The GPO was the only major rebel post to be physically taken during the week. The others surrendered only after Pearse's surrender order, carried by a nurse named Elizabeth O'Farrell, reached them. Sporadic fighting therefore continued until Sunday, when word of the surrender was got to the other rebel garrisons. Command of British forces had passed from Lowe to General John Maxwell, who arrived in Dublin just in time to take the surrender. Maxwell was made temporary military governor of Ireland.