Former Portuguese prime minister Antonio Guterres has been sworn in as Secretary General of the United Nations, becoming the ninth UN chief in the body's 71-year history.
The former UN refugee chief was elected to the top job by acclamation in the General Assembly in October.
He takes over from Ban Ki-moon on January 1.
Mr Guterres, 67, performed well in answering questions before assembly members and his executive experience as prime minister and as the UN High Commissioner for Refugees from 2005-2015 propelled him to first place among 13 candidates vying for the job in informal polls in the Security Council.
After the sixth poll, the council nominated him by acclamation and his name was sent to the assembly for final approval.
After being sworn in by General Assembly President John William Ashe, Mr Guterres addressed the 193 member nations, saying the world body must work to simplify, decentralise and make more flexible its sprawling bureaucracy.
"It benefits no one if takes nine months to deploy a staff member to the field," he said.
"The United Nations needs to be nimble, efficient and effective. It must focus more on delivery and less on process, more on people and less on bureaucracy."
The swearing-in came after the 193 General Assembly members paid tribute to Mr Ban, ending with a standing ovation for the native South Korean.
The swearing-in ceremony included the UN's top leaders and was attended by dignitaries including Burma politician and Nobel peace prize winner Aung San Suu Kyi.
US Ambassador to the UN Samantha Power lauded Mr Ban's accomplishments while in office and said she was confident Mr Guterres is the right person to replace him.
"He is the man for the job in such challenging times," she said.
The selection of a new secretary general had traditionally been decided behind closed doors by a few powerful countries.
But this year, the process involved public discussions with each candidate who was campaigning for the job.
UN chiefs are charged with promoting sustainable development, working for peace around the globe, protecting human rights and dealing with humanitarian catastrophes.
Mr Ban served two five-year terms.