MANILA  Promising to end the Philippines’ intractable corruption, Benigno S. Aquino III took office as the nation’s 15th president on Wednesday to face a welter of urgent challenges, from a swelling budget deficit to the iron grip of regional clans that have not hesitated to resort to brutality.

“You are the reason why, today, the suffering of the people will end,” Mr. Aquino, 50, told a crowd estimated by police officials at half a million. “Here, on this day, ends the reign of a government that is indifferent to the complaints of the people.”

He added: “There can be no reconciliation without justice.”

Mr. Aquino carries a powerful personal legacy of hope for Filipinos as the son of two democratic icons. His father was assassinated in 1983 as he mounted a serious challenge to the dictatorship of Ferdinand E. Marcos; his mother, Corazon C. Aquino, was swept into power with the ensuing popular uprising.

But more than 25 years later, the profound hopes for change that carried Mrs. Aquino into the presidency have dimmed, particularly during the nine years of the government of Mr. Aquino’s predecessor, Gloria Macapagal Arroyo.