Events were held across Northern Ireland yesterday to mark 100 years since Britain entered the biggest war of its history up to then.
The largest event took place at St Anne's Cathedral in Belfast city centre which was attended by First Minister Peter Robinson, Secretary of State Theresa Villiers and Prince Andrew.
During the service five candles were lit, each representing a year of the war. A list of names of Ireland's war dead was carried down from the altar and laid next to a candle, which was then lit by Prince Andrew.
Mr Robinson said he felt "deeply honoured" to attend commemoration services, and to "pay tribute to the brave servicemen and women who served and died for our freedom".
"The key themes of remembrance and reconciliation are relevant to all of us in Northern Ireland as we continue to build a peaceful and shared society," he said.
"It has often been said, but we must never forget the supreme sacrifice of so many to build a better future for us all."
A seat was reserved for Deputy First Minister Martin McGuinness at the service but he did not attend.
In Co Down the solemn centenary was marked with a gun salute over Belfast Lough.
Unionist and nationalist politicians gathered for a ceremony in which a single cannon shot was fired from Grey Point Fort in Helen's Bay, a former wartime coastal defence site.
Grey Point Fort was built as part of a wider network of coastal defensive batteries in the period 1904-07 and served as one of a pair of forts defending the mouth of Belfast Lough.
The site was decommissioned in 1956, and since 1971 has been managed by the Department of the Environment.
The fort has been preserved as a visitor attraction and features a pair of 23ft long bore Vickers guns, each of which has a 6in diameter.
SDLP Environment Minister Mark H Durkan and DUP MP Jeffery Donaldson were among those in attendance.
Grand master of the Orange Order Edward Stevenson took part in a wreath-laying ceremony at Belfast City Hall yesterday afternoon.
He spoke with pride of all those from the 36th (Ulster) and 10th, 11th and the 16th (Irish) Divisions who lost their lives.
Thousands of soldiers based in Northern Ireland also marked the date. At the headquarters of 38 (Irish) Brigade in Lisburn members of the armed forces and families attended a service during which the lights went out one by one until only a candle remained to light the stained glass window in memory of the war dead. This final light was then extinguished for a two-minutes silence.
Historians have estimated that more than 200,000 Irish-born soldiers served in the British Army and Navy from 1914 to 1918.