Britain must draw on the "courage and resolve" inspired by Winston Churchill to battle the affronts to freedom faced today, David Cameron declared on the 50th anniversary of the wartime leader's state funeral.
The Prime Minister paid tribute to "a great leader and a great Briton" after laying a wreath at the feet of the statue of his illustrious predecessor in the House of Commons Members' Lobby as part of a day of commemorative events.
Churchill is still remembered with affection by the country as a statesman, bon viveur and reformer, Mr Cameron said, but most of all as a patriot - with lessons to teach the modern world.
Prime Minister David Cameron speaks during a memorial service for Sir Winston Churchill in Westminster Hall, London
"He knew that Britain was not just a place on the map but a force in the world, with a destiny to shape events and a duty to stand up for freedom," he said in the shadow of the famous bronze sculpture of Churchill.
"That is why in 1940 - after France had fallen, before America or Russia had entered the war - he said this: 'Hitler knows that he will have to break us in this island or lose the war. If we can stand up to him all Europe may be free - and the life of the world may move forward into broad, sunlit uplands.'
"Churchill was confident that freedom and democracy would win out over barbarism and tyranny in the end... and it did.
"And with every affront to freedom in this century, we must remember that courage and resolve in the last century."
Wreaths were laid by Commons Speaker John Bercow, his Lords counterpart Baroness d'Souza and Tory MP Sir Nicholas Soames - Churchill's grandson - after a service in Parliament's St Mary's Undercroft chapel.
The three main party leaders - none of whom were born when Churchill was buried - also took part along with 17-year-old Stockton Sixth Form College student Nathania Ewruje.
The winner of the English-Speaking Union's (ESU) Winston Churchill Cup for Public Speaking, she recited from a 1955 speech of the ex-premier.
Mr Bercow praised Churchill's "recognition and fulfilment of the role of the House of Commons" as "the essence of our democracy" during an extraordinarily long parliamentary career.
And he suggested it was emblematic of the stature of the great man that he refused all pleas for him to take a seat in the Lords - adding he was very much "a green benches man".
A procession from the service was led by the Speaker's Chaplain Rose Hudson-Wilkin - ending under what is known as the Churchill Arch.
It was the former prime minister who said the entrance to the Commons chamber should be rebuilt using fragments of stone remaining after it took a direct hit from a German bomb, as a monument to the ordeals of war and the fortitude of those who stood firm.
Over decades it became tradition for MPs to touch the foot of the Churchill statue for luck when entering for debates - though the resulting shiny patch was recently repaired.
Mr Cameron - who noted that Churchill was born and is buried in the Oxfordshire constituency he now represents - said he was inspired when he first heard his voice "booming out" on records he unearthed as a boy at his grandmother's house.
As a statesman he was the first to recognise the People's Republic of China, helped realise the dream of a Jewish homeland and fought in person on the frontier in the Malakand Field Force, he noted.
"From the Battle of Omdurman to Britain's acquisition of the H bomb there stretched nearly sixty years and throughout it all he was right at the heart of events."
It was often overlooked that he was also a "reformer", he said, " who pushed for prison reform, championed old age pensions, and introduced labour exchanges for the unemployed".
And he " believed that a nation was made great not just by its military might but by how its poorest and frailest citizens were treated and that is another important part of his legacy.
"But if there is one aspect of this man I admire more than any other - it is Churchill the patriot."
He joked that he did not follow Churchill in at least one area.
"The bottles of Pol Roger in Number 10, the practice of taking his Cabinet out for lunch at the Savoy Grill. Sadly for my Cabinet, that is not quite the current regime."
Among those attending was Randolph Churchill, was said his great-grandfather would have been "surprised but thrilled" at the commemorations.
Laying a wreath at another well-known statue of the wartime leader, in Parliament Square, he said: "It was a time when people said that the nation came to a halt and people on the streets stood and paid their respects to the great man passing.
"I think he would be extremely surprised but thrilled that we are a country that thrives on its parliamentary democracy, where we have our freedoms and liberties, that we cherish them, and he would be thrilled that modern Britain has achieved so much in its own way."
He said it was also a chance to remember the heroes who fought for Britain in two world wars.
"Here we are today, 50 years on, in what Churchill referred to as the 'broad sunlit uplands', and what's sad is that we are losing that remarkable generation that served us so well in two world wars.
"It is wonderful to mark this point and remember those heroes."
Later in the day, members of Churchill's family will take part in a flotilla on the River Thames along the same route taken by his funeral.
It will include the merchant vessel Havengore, which carried his coffin, from Tower Bridge to the waters opposite the Palace of Westminster, where a wreath will be laid.
In the evening, a ceremony will be held at Westminster Abbey.
Churchill was laid to rest in the family vault at St Martin's Church in the village of Bladon, and his body was conveyed to the county on a train from London's Waterloo station to Hanborough in Oxfordshire, pulled by Southern Railway's number 34051 Battle of Britain class locomotive, named Winston Churchill in his honour.
Today the full train - the locomotive and tender, a parcel van that bore the coffin and a luxury Pullman carriage called Lydia that carried family and funeral guests - is being put back together at the National Railway Museum in York for the first time in 50 years.
During its £35,000 restoration, the locomotive was repainted with BR "Brunswick" green livery, its missing whistle was replaced and rust was repaired.
The parcel van has also been restored to its former glory and the full train is now back to its best, ready to be displayed in the museum until May 3 as part of an exhibition, Churchill's Final Journey.
Churchill's career in the Commons began in 1900 and ended just short of 64 years later, the longest in the 20th Century, and Mr Bercow said even then he only stood down at the October 1964 election "with some reluctance".
He fought 21 parliamentary elections "under no fewer that five banners including Liberal, Conservative and Independent Constitutionalist", won 16 of them and represented five different constituencies "as diverse as Dundee, Oldham and Epping", the Speaker reminded the ceremony.
"And I think it is emblematic of the whole significance and stature of the man that he never became a member of the House of Lords despite many, many invitations, opportunities and possibly even exhortations".
"He was, and this is crucial to an appreciation of him, a green benches man, a House of Commons man through and through, where he delivered thousands of speeches."
With the three party leaders standing directly before him , he said: " I think in our overall appreciation of Churchill the parliamentarian, perhaps the single most important point is Churchill's recognition that the House of Commons was not an element of our democracy; the House of Commons was the essence of our democracy."
That included a belief that during war as much as peacetime, "the cut and thrust of debate and the searing searchlight of scrutiny were vital" to test the mettle of the executive and give it legitimacy.
"The biggest tribute that we can pay to that giant .. is to hold on to the function of the House of Commons which he fought so hard and effectively to maintain and with which, by supreme personal example, he invested the institution."
Mr Bercow said he could confidently predict that Churchill would continue to inspire when the country came to mark the 100th anniversary.
The wreaths carried hand-written cards.
Mr Cameron's said: "Britain was so incredibly fortunate that in our hour of greatest need there came forward one of our greatest ever statesmen. 50 years on the light has not dimmed."
Deputy Prime Minister Nick Clegg wrote: "In memory of a man who defended our nation, defeated fascism, led the free world and has made generations of his fellow citizens proud to be British."
And Labour leader Ed Miliband's message read: "With gratitude, respect and admiration for the leadership you showed during Britain's darkest hour. Our country will always remember the hope you gave and the courage you inspired."
Randolph Churchill and Celia Sandys, the great-grandson and granddaughter of former Prime Minister Sir Winston Churchill, lay a wreath at his statue on Parliament Square in London to mark the 50th anniversary of his funeral.