US President Barack Obama on Tuesday basked in the lavish royal pageantry of a state visit to Britain, given an extra dash of glamour by a brief encounter with Prince William and his bride Catherine.
But the 24-hour demands that follow a US president everywhere shadowed the London pomp, as Obama took time out to say he was "heartbroken" at the toll of vicious tornados which ripped across the US midwest, killing 116 people.
Queen Elizabeth II was officiating at the ceremonial portion of Obama's two-day visit, also aimed at spurring a push for democracy in the wake of the Arab Spring, with an official welcoming event at Buckingham Palace.
Earlier, Obama and his wife First Lady Michelle Obama were to meet William and Catherine, now known as the Duke and Duchess of Cambridge, less than a month after watching their fairytale wedding on television.
Later, the queen will fete Obama with all the trappings of a state dinner and the Obamas will stay in the palace's sumptuous Belgian suite, last used by William and the former Kate Middleton on the night of their April 29 wedding.
The serious political business of the visit happens on Wednesday, and Obama and British Prime Minister David Cameron limbered up for their talks with a vow to support those risking their lives for reform in the Arab world.
"We will not stand by as their aspirations get crushed in a hail of bombs, bullets and mortar fire. We are reluctant to use force, but when our interests and values come together, we know we have a responsibility to act," they wrote.
"We will stand with those who want to bring light into dark, support those who seek freedom in place of repression, aid those laying the building blocks of democracy," they said in an article in the Times newspaper.
The declaration of intent comes less than a week after Obama spelled out his long-awaited response to the tumult which has ousted autocrats and reshaped nations in the Middle East and North Africa.
In a semantic riff on the so-called "special relationship" between the United States and Britain, Obama and Cameron also heralded a new "essential relationship" between the countries.
The NATO mission in Libya, backed by US logistics, but led by Britain and France, is also likely to be a key issue when Cameron and Obama meet for an informal chat on Tuesday, ahead of formal talks the next day.
Britain is reportedly planning to send Apache helicopters to attack Moamer Kadhafi's forces in Libya, in a bid to unlock the stalemate there.
Diplomatic and military maneuvering is heating up over Libya ahead of the G8 summit in France, Obama's next stop on a four-nation European tour which started with a journey to his ancestral roots in Ireland and also takes in Poland.
Before enjoying the pomp and circumstance of a royal visit, Obama took care of domestic concerns, with on-camera remarks on a tornado tragedy that is dominating media coverage back home in the United States.
He said he was "heartbroken" by the death toll wrought by one killer storm in Joplin, Missouri, where 116 people are so far confirmed dead.
Mindful that no US leader can afford to hobnob with royals abroad while a disaster unfolds at home, Obama said he would visit the disaster zone on Sunday, hours after returning from Europe.
But for Tuesday, the main focus was on the royals and the eye-catching meeting with William and Catherine, fresh from their honeymoon in the Seychelles.
The two couples were due to meet at Buckingham Palace for about 10 minutes just before noon, before the formal ceremonial welcome for the Obamas in the palace garden.
The Obamas were not invited to the wedding because William is not heir to the throne -- he is second in line after his father, Prince Charles -- so the meeting will be the president's first chance to congratulate the couple in person.
After the formal welcome and lunch, the queen will show the Obamas US-related items in the Royal Collection and the Royal Archives.
The Obamas were also due to lay a wreath at the tomb of the Unknown Warrior in Westminster Abbey -- where Catherine deposited her wedding bouquet in a tradition begun by the late Queen Elizabeth, the Queen Mother.
The queen struck up a visibly close friendship with Michelle Obama when the Obamas visited Britain for the first time in 2009, with both women putting their arms around each other in a highly unusual gesture.
On Wednesday, the president is granted the rare honour of addressing both houses of the British parliament.