The Queen led the nation in honouring members of the armed forces killed in conflict as Remembrance Sunday services took place around the country today.

The monarch laid the first wreath at the Cenotaph in London's Whitehall to commemorate those who have made the ultimate sacrifice in the decades since the First World War, bowing her head after paying her respects.

Senior royals, including Second World War veteran the Duke of Edinburgh, the Prince of Wales and the Duke of Cambridge joined politicians, military leaders, veterans and serving personnel in laying wreaths of poppies at the monument.

Prime Minister David Cameron described this year's Remembrance Sunday as "particularly poignant" as 2014 marked the 100th anniversary of the start of the First World War, the 70th anniversary of the D-Day landings and the end of Britain's 13-year conflict in Afghanistan.

Millions across the UK fell silent in tribute to those lost in war, joining the crowds gathered in central London who stood in a moment of quiet contemplation as Big Ben struck 11am.

Amid heightened fears of a terror attack, there was a large police presence at the Cenotaph in Whitehall, with a helicopter hovering above the site before the ceremony.

Scotland Yard said it had an "appropriate and proportionate" policing plan in place for the event, after four men were arrested in connection with an alleged Islamist terror plot on British soil on Thursday.

At the heart of the service was a two-minute silence, marked at the beginning and end by the firing of a round by the King's Troop Royal Horse Artillery, using a 13-pounder First World War gun.

At the end of the silence, buglers of the Royal Marines sounded the last post.

In cool and overcast conditions, the royals and dignitaries then laid their wreaths at the Cenotaph.

Mr Cameron was first after the royals to do so, followed by Deputy Prime Minister Nick Clegg and Labour leader Ed Miliband.

Former prime ministers Sir John Major, Tony Blair and London mayor Boris Johnson also took part in the ceremony.

The Duchess of Cambridge and Duchess of Cornwall watched from the Foreign Office balcony.

A young choir boy collapsed at the Cenotaph yards from where the royal family were standing.

As the national anthem was sung, medics rushed to treat the boy who was helped up and led away through the crowd.

The Queen wore a long black coat and hat and carried a black handbag during the service, while the Duke of Edinburgh, who joined the Royal Navy in 1939, wore the uniform of an Admiral of the Fleet for the ceremony.

Forty-six high commissioners from Commonwealth countries each laid a wreath before the Irish ambassador to the UK, Dan Mulhall, left a floral tribute.

It was the first time a diplomat from the Irish Republic has laid a laurel wreath at the London Cenotaph - the latest in a line of symbolic gestures by both the UK and Ireland aimed at putting their troubled history behind them.

Some 200,000 Irish-born soldiers from north and south of the island served in the First World War, with around 50,000 losing their lives.

Following the wreath-laying, the Bishop of London, the Right Reverend Richard Chartres, conducted a short service in his role as Dean of HM Chapels Royal.

In Scotland, First Minister Alex Salmond and Scottish Secretary Alistair Carmichael attended a service at Edinburgh's St Giles' Cathedral, while Deputy First Minister Nicola Sturgeon was in Glasgow for a ceremony in George Square.

Remembrance Sunday events were also held in Dundee, Aberdeen and elsewhere.

Mr Salmond said: "The people of Scotland will always honour, with respect and appreciation, the memory of those who made the ultimate sacrifice on our behalf.

"Scotland, in common with so many other nations, suffered an appalling loss of life in the Great War, and its effects on Scottish life were profound and long-lasting. Not one single community was untouched by the conflict, and rural Scotland suffered particularly severe losses.

"It takes an incomparable event to bring a whole nation to a halt - united in remembrance, reflection and gratitude - and there is no more profound an event than a conflict that saw over a hundred thousand sons, fathers, uncles never return from the front line.

"Today we stand with members of the German navy to remember and pay tribute to the fallen; a true symbol that in conflict there is always the resounding hope for peace and reconciliation."

Mr Carmichael said: "This year's Remembrance Sunday service has particular significance in a year when Scotland has been front and centre of the commemorations to mark 100 years since Britain entered the First World War and 70 years since we stood shoulder to shoulder with our allies on the beaches of Normandy.

"Today, Scotland's support for the UK armed forces and the wider armed forces community is unwavering. We are all indebted to their bravery, courage and selflessness serving in the line of duty."

Interim leader of the Scottish Labour Party, Anas Sarwar MP, said: "As we mark the 100th anniversary of the outbreak of the First World War we must not forget a generation of Scots whose suffering was immeasurable, sacrifice incalculable and whose legacy we must forever honour.

"From the fields of Flanders to the plains of Afghanistan, we pay tribute to those brave men and women, across the generations, who have sacrificed all to defend our values and freedoms across the world."