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, 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 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.