The world hailed the dawning of the Barack Obama era at the White House but the leaders the new United States president will have to deal with also warned him of the difficulties ahead.
Millions followed Mr Obama's inauguration worldwide, with parties held from London to Sydney. Thousands danced in the Kenyan village where his father was born, while his former classmates celebrated at his former school in Indonesia.
Underscoring the huge show of global faith in Mr Obama's leadership, French President Nicolas Sarkozy said: "We are eager for him to get to work so that with him we can change the world."
British Prime Minister Gordon Brown hailed Obama as a "man of great vision" and his arrival as a "new chapter in both American history and the world's history".
"He's not only the first black American president but he sets out with the determination to solve the world's problems," Mr Brown said.
New Zealand Prime Minister John Key said no incoming American president has faced greater expectations and challenges than Mr Obama, and New Zealand looked forward to developing a strong working relationship with his administration.
Australian Prime Minister Kevin Rudd said Mr Obama's message of hope resonated around the globe. "This is a great moment, not just for the people of America, but for all people around the world that believe in democracy, who believe in freedom and who believe in progress."
Japan's Prime Minister Taro Aso also vowed to work with Obama to boost "peace and prosperity in the Asia-Pacific region and the world".
South Africa's former president and anti-apartheid icon Nelson Mandela hailed Obama as "new voice of hope" in a letter to the new leader. "Your election to this high office has inspired people as few other events in recent times have done," he wrote.
Pope Benedict sent a message to Obama calling on him "to promote understanding, cooperation and peace" among nations.
Warning for new president
However, some leaders sought to temper the high expectations. German Chancellor Angela Merkel wished Mr Obama "the best of luck" but gave the latest in a series of warnings to the new US president.
Ms Merkel said Obama would not sway Germany to add further to its committed 4,500 troops in Afghanista and expressed scepticism that Mr Obama's plan to seek direct talks with Iran - which many countries accuse of seeking nuclear weapons - would work.
French Foreign Minister Bernard Kouchner said he "shared the admiration and emotion of the whole world" for Mr Obama. But he added: "I think we should not expect him to immediately solve all America's problems, nor ours. Barack Obama does not have a magic wand."
Mr Obama inherits an economy in crisis, wars in Afghanistan and Iraq and a conflict in the Middle East where the United States has a key role. Iran's contested nuclear programme will also be among his major challenges.
Israeli Prime Minister Ehud Olmert said he hoped their two countries would be "full partners in promoting peace and stability in the Middle East".
Iranian Foreign Minister Manouchehr Mottaki said his country would await the "practical policies" of the new US president before passing judgment on him.
China offered a nervous welcome to the new president, expressing concern over the direction he may take in Sino-US ties after the improvements seen under his predecessor in their once volatile relationship.