Ashraf Ghani has been sworn in as Afghanistan's President in a ceremony at the Presidential Palace in Kabul.
It came after six months of deadlock amid a bitter dispute over electoral fraud and a recount of votes.
Under a US-brokered unity deal, Mr Ghani takes over the Presidency and runner-up Abdullah Abdullah can nominate a figure with Prime-ministerial powers.
The Taliban have described the deal as a "US-orchestrated sham" but Mr Ghani hailed it as a "big victory".
Mr Ghani took an oath to abide by the constitution and other laws at the swearing-in ceremony.
He praised the country's "first democratic transfer of power" and has also spoken warmly of his rival, and now partner in government, Mr Abdullah.
In a speech as the ceremony began, outgoing President Hamid Karzai, who has been leader since the US-led invasion in 2001, called for people to support the new government.
Reports from Kabul say security in the capital is tight, with few people on the streets and shops closed.
Sources say the first thing the government is expected to do is to sign a deal that will see US troops remain in Afghanistan after the end of this year - a move opposed by Mr Karzai.
Militants attacked a government compound in the eastern province of Paktia Monday, officials say.
Meanwhile, a roadside bomb exploded on the airport road in Kabul. No one was hurt.
On Friday the Taliban overran a strategic district in another eastern province, Ghazni, highlighting some of the many challenges facing Mr Ghani and his security forces.
US Secretary of State, John Kerry helped to broker a comprehensive audit of all eight million votes in the election earlier this year after the results were disputed.
The audit was completed this month but the final tallies and the official result have not been made public amid fears over unrest.
Afghanistan's election commission confined itself to declaring Mr Ghani the winner in a statement earlier this month.
Both sides had accused the other of fraud following the election and months of uncertainty have damaged the economy and heightened insecurity.