Today's service was coinciding with a similar event in Westminster Abbey also to commemorate the outbreak of the First World War.
Earlier, an invitation-only service in Glasgow Cathedral was being attended by Prince Charles.
It was being led by Dr Laurence Whitley and was honouring the present day Commonwealth's contribution to the war.
The commemoration was being shown live in George Square for members of the public who had applied for tickets.
It was being followed by a procession to the Cenotaph in the Square for a wreath-laying service and march past at 11.30am.
The event was due to finish at 12.15pm.
Solemn services of remembrance were also being held in London and Belgium.
The evening service, which begins at 10pm, is open to any member of the public who wishes to attend.
Culture Secretary Sajid Javid, who is leading the government programme, said: "A hundred years on, the sheer scale and sacrifice of the First World War demand remembrance.
"Breaking with tradition, we are marking the start as well as the end of the conflict so the anniversaries of the war will improve our understanding of its causes, conduct and consequences."
Glasgow Lord Provost Sadie Docherty, said: "It is a great honour for Glasgow to host the UK and Commonwealth service to mark the start of the First World War.
"Around 200,000 Glaswegian men volunteered to go to war, 18,000 of whom never returned.
"Glasgow women also left our city to work in front-line hospitals across Europe and beyond as doctors, nurses, orderlies and cooks helping to treat the many soldiers wounded in the fighting.
"On the home front, other Glaswegian women took over the jobs that the men left behind, building ships and munitions for the war effort, as well as driving trams, fire engines and ambulances.
"This event gives Glasgow an opportunity to reflect on our city's great contribution and how at the end of four years of war the lives of everyone who lived through that time was changed forever.
"For these reasons it is essential to commemorate, remember and learn from and about the lives of Glaswegians during 1914-1918 and we hope that the centenary commemorations will encourage people to investigate their own family and community war stories."