Royalty, political leaders and families of the fallen will unite in Belgium and the UK today in marking 100 years since Britain entered the First World War.

At 11pm on August 4, 1914, Britain declared war on Germany, ushering in four years of darkness, despair and appalling tragedy.

Until the armistice was signed on November 11, 1918, millions of lives were lost, including 750,000 British and Commonwealth soldiers, in what was the bloodiest conflict the world had known.

The world's last known First World War service member, Florence Green, died aged 110 in King's Lynn, Norfolk, in February 2012.

Today, on a national day of commemoration, events marking the anniversary of the start of the Great War will be held in London, Glasgow and Belgium, will begin a four-year Government-led programme of remembrance.

Tonight the Duke and Duchess of Cambridge and Prince Harry will be among 500 guests who will attend a twilight ceremony at St Symphorien military cemetery near Mons.

Prime Minister David Cameron, Irish president Michael D Higgins, the King and Queen of Belgium and German president Joachim Gauck will also attend.

Mr Cameron said: "A hundred years ago today Britain entered the First World War and we are marking that centenary to honour those who served, to remember those who died, and to ensure that the lessons learnt live with us forever.

"It is right to remember the extraordinary sacrifice of a generation and we are all indebted to them because their most enduring legacy is our liberty."

Helen Jones, 57, from Westwoodside, north Lincolnshire, plans to lay flowers on the grave of her great-uncle Private George Bellamy of 2nd Battalion the Royal Irish Regiment, who died on August 26 1914, just days into the Battle of Mons.

The cemetery is also the final resting place for the first and last British soldiers - and the last Commonwealth troops - to die on the Western Front.

The event will mainly be narrated by historian Dan Snow and will include readings, music and poetry capturing the history of the site.

In Glasgow, the Prince of Wales will attend a service at the city's cathedral, followed by a wreath-laying service and march-past at the Cenotaph in George Square.

Lord Provost of Glasgow, Councillor Sadie Docherty, said: "I hope people will be inspired over the coming four years to learn more about their own family and community war history."

In London at 10pm - an hour before war was officially declared 100 years ago - a service of solemn commemoration will be held at Westminster Abbey.

Key figures there will include the Duchess of Cornwall, Deputy Prime Minister Nick Clegg, Labour leader Ed Miliband and Metropolitan Police commander Sir Bernard Hogan-Howe.

Mr Clegg said: "Sixteen million people perished in World War I. It's an almost unimaginable number of people who died in a war which still shapes the world as it is today."

Mr Miliband said: "Young men from across Britain served alongside soldiers from across the world - from the Indian sub-continent to Africa, from Australia to the Caribbean.

"We must also remember those who served their country in other ways, from nurses who risked their lives on the Western Front to those who played their part on the Home Front."

The service will include the gradual extinguishing of candles, with an oil lamp extinguished at the tomb of the unknown soldier at 11pm - the exact hour war was declared.

In the same hour, the nation has been urged to switch off lights in places of worship, public buildings, workplaces and homes, and leave one light burning as a symbol of hope in darkness.

The project is a reference to then-foreign secretary Sir Edward Grey's famous remark on the eve of the outbreak of war, when he said: "The lamps are going out all over Europe; we shall not see them lit again in our life-time".

In a foreword to the service, the Dean of Westminster Abbey, the Very Rev Dr John Hall, said it aimed to focus attention on the last moment of peace before Britain declared war on Germany.

He wrote: "The bloody conflagration lay ahead; tonight we catch glimpses of people's anticipation of what was to come.

"But we also have an opportunity to reflect on the attitudes of those preparing for war: some, frankly, relished the prospect; others dreaded it.

"Most were stumbling into the darkness, increasingly bound by the chains of their own and others' making, their hope of avoiding war ever fading."

Within weeks of Britain declaring war on Germany, the two nations' forces clashed outside Mons, leading to some 1,600 British casualties and 2,000 German.

St Symphorien, which is maintained by the Commonwealth War Graves Commission (CWGC), is the final resting place for 229 Commonwealth and 284 German troops.

The CWGC's director-general Brian Davidson said: "St Symphorien is a uniquely fitting place for us to gather in a spirit of common remembrance.

"On land donated by a Belgian, in a cemetery first built by the German Army and now cared for by the CWGC, the fallen from both sides of the conflict lie together at peace."

Elsewhere, Anglican churches around the UK will hold similar services, including at St Anne's Cathedral in Belfast and at Llandaff Cathedral in Cardiff.

And acts of reflection will be held in places of worship of other denominations and faiths, and Catholic churches held special Masses yesterday.

The Metropolitan Police will be among forces up and down the country remembering fallen comrades who participated in the war effort and did not return.

Deputy commissioner Craig Mackey said: "Their sacrifice helped to shape the world we inherited from them and the freedoms we so often take for granted."

During the four-year project each of the 430 men awarded the Victoria Cross - the British Empire's highest medal for military valour - in the war will have their names enshrined in a paving stone in their home town.

Each stone will be installed on the 100th anniversary of the action for which the medal was issued, and will include a digital sign or QR (Quick Response) code giving people more information.

The first two, set to be laid this month, will commemorate Charles Garforth of Willesden Green in London and Sidney Godley of East Grinstead, West Sussex.

The scheme has also been extended to see foreign combatants who won the VC fighting for Britain during the war honoured.

The Government has contributed more than £50 million of funding to support commemoration activities.

This includes an educational programme giving two students and a teacher from every state school in the country the chance to visit First World War battlefields and research local people who fought in the war.

Some £5 million, funded by Libor fines, will also be used to help conserve, repair and protect First World War memorials and graves across the UK and overseas.

There will also be a Christmas Day Truce football match on the battlefields of Flanders to mark the anniversary of the historic match between British and German soldiers on Christmas Day 1914, while the Premier League is donating a state-of-the-art floodlit football pitch in Ypres.