US President Barack Obama was welcomed to Buckingham Palace by Queen Elizabeth today on his first state visit to Britain.
The American leader has a busy round of political talks and meetings during his high profile three-day stay, as well as a lavish state banquet and a meeting with the newly-married Duke and Duchess of Cambridge.
The state visit is only the third by a US president to the UK in 100 years and comes at a time of close co-operation between Britain and America on issues ranging from Libya to Afghanistan, counter-terrorism and the Middle East peace process.
In a joint article written for The Times, Mr Obama and Prime Minister David Cameron declared: "Ours is not just a special relationship, it is an essential relationship - for us and for the world."
At the red carpeted Grand Entrance of the Palace, the President and the First Lady Michelle Obama were greeted by the monarch and the Duke of Edinburgh.
The Obamas formed a strong bond with the royal couple following their visit during the G20 summit two years ago when, after a photocall for leaders, the monarch and Mrs Obama were seen putting their arms around each other's backs.
Sharing an interest in countryside, gardening and clothes, the Queen and the President's wife kept in touch by letter and phone and Mrs Obama even returned for a private trip to the Palace later in the year.
The President told the BBC ahead of his visit that the Queen symbolised the "best of England" to the whole world and described the monarch and her husband as "extraordinarily gracious people".
Mr Obama flew into Stansted airport ahead of schedule last night to avoid the possibility of disruption to air travel from the ash cloud from an erupting Icelandic volcano.
Amid tight security, his motorcade swept into the Queen's London home from the US ambassador's residence in London, Winfield House in Regent's Park, joined by the heir to the throne the Prince of Wales and the Duchess of Cornwall.
Before he left for the palace, President Obama spoke to reporters, expressing his sorrow about the tornado devastation back home in Missouri, Minnesota and around the Midwest.
He said he would travel to the affected region in Missouri on Sunday to assure people "the whole country is going to be behind them".
He said: "We have been heartbroken by the images we have seen ...
"The devastation is incomparable ... So far we know over 100 people lost their lives ...
"Our thoughts and prayers are with the families who are suffering at this moment."
One a half-mile-wide tornado killed at least 116 people as it blasted the Missouri town of Joplin off the map and tore through its hospital earlier this week.
It was the deadliest single twister in the US in nearly 60 years and the second major tornado disaster in less than a month.
The Obamas are staying as guests of the Queen and will occupy the Palace's opulent Belgian suite, as is customary for visiting foreign heads of state.
It was there that William and Kate spent their first night as a married couple after partying into the early hours following their wedding.
The extravagant suite of rooms is on the ground floor of the north-facing garden front and was first decorated for Prince Albert's uncle Leopold I, first king of the Belgians
In 1936, King Edward VIII lived in the rooms briefly before he abdicated the throne.
Following the ceremonial welcome, the visitors will join the Queen and Philip for lunch and then view a special exhibition in the royal picture gallery, before a wreath-laying visit to Westminster Abbey, a trip to Downing Street and a state banquet this evening.
Tomorrow will be devoted to politics, with talks between Mr Obama and Mr Cameron at 10 Downing Street, followed by an address to both Houses of Parliament, in which the president is expected to say that the US has no closer ally in the world than Britain.
The two leaders are also expected to drop in on a barbecue being hosted by their wives, Michelle and Samantha, for families of military personnel involved in joint UK-US missions overseas.
Security round Winfield House was tight this morning, with a huge police and secret service presence.
Set in 12 acres of grounds in Regent's Park, Winfield House has the largest private garden in or close to central London after Buckingham Palace.
It has been the official residence of the US ambassador to the United Kingdom since 1955, and the Obamas stayed there on their last visit to London from March 31 to April 2 2009.
Charles and Camilla arrived at about 11.25am privately in a Rolls-Royce - one of the state cars from the royal mews.
Their meeting with the Obamas was completely private and media were given just a glimpse of the two couples as they left Winfield House to bright sunshine.
The Duchess of Cornwall wore a cream silk dress and a cream raw silk jacket by Anna Valentine, with a cream feathered hat by Philip Treacy, while the first lady wore a turquoise dress and pink bolero jacket.
All four left in the "Beast" - the president's armoured limousine - to be taken to Buckingham Palace for their meeting with the Queen.
Thousands of tourists who had gathered for the changing of the guard ceremony watched as the convoy of more than 15 large limousines swept into the palace's quadrangle accompanied by an ambulance - a precautionary measure.
The Beast slowly stopped in front of the grand entrance and the substantial door was pulled open by a US security official.
President Obama was sitting with Charles on his left and opposite the US leader was his wife, who had Camilla beside her.
The Obamas stepped out on to the steps of the palace's grand entrance where the Queen and the Duke of Edinburgh were waiting to greet them.
The US leader gave the Queen a warm double handshake, clasping her hand with his for a few moments as they exchanged words.
He also greeted Philip, telling him "Nice to see you sir", while his wife shook the Queen's hand and the Duke's.
The foursome posed for a few moments for the photographers, including many from the US, before heading inside the royal residence, with the two heads of state leading the way and Charles and Camilla following at the back.
The Obamas were given around 10 minutes to settle themselves into their palace suite after arriving.
They were then taken to the 1844 room to meet newlyweds William and Kate, where discussions were likely to have covered their recent wedding and honeymoon.
Before heading outside into the palace's picturesque garden for the ceremonial welcome, senior members of the Queen's household were introduced to the Obamas in the Bow room.
Among them were the Queen's private secretary Christopher Geidt, her treasurer Sir Alan Reed, master of the household Air Vice Marshall David Walker, and the Lord Chamberlain Earl Peel.
Others who met the president and his wife included Brigadier Archie Miller-Bakewell, Philip's private secretary, Lieutenant Colonel Andrew Ford, comptroller of the Lord Chamberlain's Office, and a number of ladies-in-waiting to the Queen including Virginia Ogilvy, Countess of Airlie.
HIS voice, that velvet-rich, immense voice, rose and soared and his words were carried by the wind down Dame Street, around College Green, up towards Christchurch, over the sea of people who had come to hear him.