Ban, 62, pledged that after he officially takes up his post Jan. 1, he would try to restore trust in the institution tainted by scandal and management lapses and to bridge divisions between rich and poor nations.
"I look forward with a mixture of awe and enthusiasm to taking up my duties as secretary-general of the United Nations," he said after being sworn in.
Before Ban took his oath in the vaulted General Assembly hall, representatives from 192 countries gave a thundering ovation to outgoing Secretary-General Kofi Annan, 68, who will step down after 10 years in office.
Although Annan brought international spotlight to the issues of human rights and development, saying they deserve as much attention as matters of war and peace, his tenure also was blemished by findings of corruption and mismanagement in the $64-billion Iraq oil-for-food scandal.
Annan's grand vision of reforming the institution was scuttled by developing countries, which viewed the changes as a power grab by U.N. bureaucrats and by the United States, which proposed a more stringent plan.
While praising Annan's "high ideals, noble aspirations and bold initiatives," Ban hinted at a shift from his predecessor's approach.
"The time has come for a new day in relations between the secretariat and member states," he said. "The dark night of distrust and disrespect has lasted far too long."
He also said he would strive to "lead by example" to set high ethical standards at the world body, a statement also seen by diplomats as a subtle swipe at Annan and the scandals that have been exposed at U.N. agencies under his watch.
Ban, whose career as a South Korean diplomat kept him largely out of the spotlight, has said that his modest and unassuming demeanor should not be mistaken for weakness or indecision.
He told the press that he would not evade questions, as he was reputed to do as foreign minister.
"Your colleagues in Korea may have dubbed me the 'slippery eel' because I was too charming for them to be able to catch me," he said. "But I promise today that I can be a pretty straight shooter when I need to."
Addressing concerns that he would be more of an administrator than a leader, Ban said that he was willing to personally engage to find solutions to the standoffs involving Darfur, Iran and North Korea.
"The suffering of the people of Darfur is simply unacceptable," he told reporters. He said he would push leaders in Sudan to allow U.N. peacekeepers to protect the people of Darfur if the government could not halt the violence.
He also said he would meet with Iranian leaders "wherever and when" to discuss their denial of the Holocaust, an attitude he called "not acceptable." He told a news conference that among the many crises in the world, the conflict between Israelis and Palestinians was at the core of tensions in the Middle East and "the most serious issue with which we must deal." He said he planned to revitalize the "road map" peace initiative and to speak soon with leaders in the region.
But Ban left most of his concrete plans and policies vague, saying he would like to resolve issues in the Middle East, Lebanon, Iran and Iraq in each case "in a peaceful way through dialogue."
The new secretary-general also made it clear that despite all his calls for dialogue, he won't be doing it in French.
French is the second official working language of the United Nations, and Ban's resume says that he speaks it -- an unstated requirement for his new post. French President Jacques Chirac interviewed Ban to make sure he could pass muster as a Francophone.
But when a Canadian reporter asked Ban in French why it should be one of the U.N.'s working languages, he didn't understand.
"If you speak 'lentement en francais' [slowly in French] I will do my best," he said.
In the end, he relied on an interpreter and answered the question in English, earning him a new nickname from the press corps: "Franco-phony."