In Vienna, former Nobel Peace Prize winner Mohamed ElBaradei, director general of the International Atomic Energy Agency, said Obama has already provided outstanding leadership in the effort to prevent nuclear proliferation.
"In less than a year in office, he has transformed the way we look at ourselves and the world we live in and rekindled hope for a world at peace with itself," ElBaradei said. "He has shown an unshakable commitment to diplomacy, mutual respect and dialogue as the best means of resolving conflicts. He has reached out across divides and made clear that he sees the world as one human family, regardless of religion, race or ethnicity."
Still, some said the award came too soon, in light of the lack of tangible progress toward the vital goals of bringing peace to the Middle East, persuading Iran to give up its nuclear ambitions and improving relations with North Korea.
"The award is premature," said Eugene Rogan, director of the Middle East Center at Oxford University in England. "He hasn't done anything yet. But he's made clear from the start of his presidency his commitment to promote peace. No doubt the Nobel committee hopes the award will enhance his moral authority to advance the cause of peace while he's still president."
Massimo Teodori, one of Italy's leading experts of U.S. history, said the Nobel decision is a clear rejection of the "unilateral, antagonistic politics" of Obama's predecessor, George Bush.
"The prize is well deserved after the Bush years, which had antagonized the rest of the world," Teodori said. "President Obama's policy of extending his hand has reconciled the United States with the international community."
Reaction was far more muted in Pakistan, where many have criticized U.S. policies.
In Pakistan's central city of Multan, radical Islamic leader Hanif Jalandhri said he was neither happy nor surprised by Obama's award.
"But I do hope that Obama will make efforts to work for peace, and he will try to scrap the policies of Bush who put the world peace in danger," said Jalandhri, secretary general of a group that oversees 12,500 seminaries. "This prize has tripled Obama's responsibilities, and we can hope that he will try to prove through his actions that he deserved this honor."
Associated Press Writers Abisalom Omolo in Kogelo, Kenya, Celean Jacobson in Johannesburg, Alessandra Rizzo in Rome, Matti Friedman in Jerusalem, Rahim Faiez in Kabul and Khalid Tanveer in Multan, Pakistan contributed to this story.