Ashraf Ghani has been sworn in as Afghanistanâ€™s new president, replacing Hamid Karzai in the countryâ€™s first democratic transfer of power.
The Kabul ceremony followed six months of deadlock amid a bitter dispute over electoral fraud and a recount of votes.
Under a US-brokered unity deal Mr Ghani shares power with runner-up Abdullah Abdullah who becomes chief executive.
Two bombs killed at least 15 people as Mr Ghani was sworn in. The Taliban call the deal a â€œUS-orchestrated shamâ€.
One blast near Kabul airport killed at least seven people. A second attack in eastern Paktia province left another eight dead, officials said.
The Taliban said they carried out both attacks.
At the swearing-in ceremony attended by up to 100 dignitaries at the presidential palace in Kabul, Mr Ghani took an oath to abide by the constitution.
He said he would work for long-term peace, promised to tackle corruption and said constitutional changes were needed.
â€œSecurity is a main demand of our people, and we are tired of this war,â€ Mr Ghani said. â€œWe ask opponents of the government, especially the Taliban and Hezb-e-Islami [another militant group], to enter political talks.â€
Analysis: David Loyn, BBC News, Kabul
In his long first speech after being sworn in as the new Afghan president, Ashraf Ghani promised reform, development, an end to poverty, measures against corruption, and a clean-up of the judiciary.
But he knows that he can do nothing without security. In a year when the Taliban are conducting their biggest offensives since they fell from power in 2001, he said Afghan people were tired of war and wanted peace.
He talked about the crisis in Syria and Iraq, and said that Afghanistan was an Islamic country that understood about jihad after Islamic guerrillas defeated the Russians in the 1980s. He said the country did not need any lessons from foreign fighters.
The former World Bank economist has ambitious plans for change, but now needs to make his coalition work, with his defeated election rival Abdullah Abdullah.
Mr Ghani praised the countryâ€™s â€œfirst democratic transfer of powerâ€ and also spoke warmly of his rival, and now partner in government, Dr Abdullah. His first act as president was to sign the decree creating the new role of chief executive.
Dr Abdullah, who takes on the job which has prime-ministerial powers, said the two leaders would work together â€œfor a better future with trust and honestyâ€.
Earlier outgoing President Hamid Karzai, who has been leader since the US-led invasion in 2001, called for people to support the new government.
Security in the capital was tight, with few people on the streets and shops closed.
- Previously an academic in the US, and worked for the World Bank
- Served as finance minister from 2002-2004 under President Hamid Karzai
- He fell out with President Karzai in 2004 and became Chancellor of Kabul University, where he was seen as an effective reformer
- He has been a vocal critic of the way international aid money has been wasted in Afghanistan
- He has a well-known reputation for having a quick temper
On Tuesday, the new governmentâ€™s first act is expected to be signing a deal that will see US troops remain in Afghanistan after the end of this year â€“ a move previously opposed by Mr Karzai. All Nato combat troops pull out this year.
Underlining the security challenges facing the countryâ€™s new leaders, officials said a suicide bomber had attacked a checkpoint on the Kabul airport road leading to the US embassy as the palace ceremony was being held.
Four members of the Afghan security forces and three civilians were killed, and a number of other people were wounded, interior ministry spokesman Sediq Sediqi said.
In Paktia, a suicide car bomb attack on a government compound killed at least eight people, officials say. The raid in Zurmat district left a number of security personnel dead. Officials say four attackers were also killed.
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.
Following the election earlier this year, US Secretary of State John Kerry helped broker a comprehensive audit of all eight million votes 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.
Afghanistanâ€™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.