At midday a small team of Volunteers and Fianna 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. The explosion was not loud enough to be heard in the city. At the same time, the Volunteer and Citizen Army forces throughout the city moved to occupy and secure their positions.
Seán Connolly's unit made an assault on Dublin Castle, shooting dead a police sentry and overpowering the soldiers in the guardroom, but did not press home the attack. The Under-secretary, Sir Matthew Nathan, who was in his office with Colonel Ivor Price, the Military Intelligence Officer, and A. H. Norway, head of the Post Office, was alerted by the shots and helped close the castle gates.
The rebels occupied the Dublin City Hall and adjacent buildings. Mallin's detachment, which was joined by Constance Markiewicz (Countess Markiewicz), occupied St. Stephen's Green, digging trenches and commandeering vehicles to build barricades.
They took several buildings, including the Royal College of Surgeons, but did not make an attempt on the Shelbourne Hotel, a tall building overlooking the park. Daly's men, erecting barricades at the Four Courts, were the first to see action. A troop of the 5th and 12th Lancers, part of the 6th Cavalry Reserve Regiment, was escorting an ammunition convoy along the north Quays when it came under fire from the rebels. Unable to break through, they took refuge in nearby buildings.
The headquarters battalion, led by Connolly, marched the short distance to O'Connell Street. They charged the GPO, expelled customers and staff, and took a number of British soldiers prisoner. Two flags were hoisted on the flag poles on either end of the GPO roof: the tricolour at the right corner at Henry Street and a green flag with the inscription 'Irish Republic' at the left corner at Princess Street. A short time later, Pearse read the Proclamation of the Republic outside the GPO.