Ashraf Ghani has been inaugurated as the new president of Afghanistan at a ceremony in the presidential palace marking the end of Hamid Karzai's 13-year rule since the fall of the Taliban regime in 2001.
"I will abide by the constitution and other laws and protect their implementation," Ghani said in an oath administered by the chief justice.
Ghani became president of Afghanistan in a peaceful transition after a six-month election season that ended with marathon negotiations that gave the newly created position of chief executive to his challenger Abdullah Abdullah.
Ghani entered the presidential palace wearing a dark black turban popular in the country's south.
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His electoral challenger Abdullah, who is slated to fill the newly created position of chief executive, sat two seats away, with Karzai anchoring the spot in between.
Karzai wore a wide smile as he greeted his presidential guards upon entering the palace.
Karzai has said he is glad to be stepping down after more than a decade of what the US ambassador recently said was one of the most difficult jobs in the world.
The inauguration caps a nearly six-month election season that began when ballots were first cast in April.
A runoff election in June between Ghani and Abdullah stretched on for weeks as both sides levelled charges of fraud.
The United Nations helped carry out what it said was the most thorough recount in its history, a count that reduced Ghani's vote percentage from 56 per cent to 55 per cent, but still gave him the win.
But the real power struggle was taking place in marathon talks between the two sides, often brokered by US Secretary of State John Kerry and other US officials.
The political deal the sides agreed to created the new position of chief executive that Abdullah will now fill.
US officials have said they expect Ghani to sign a security agreement with the US shortly after his inauguration to allow about 10,000 US troops to stay in the country after the international combat mission ends on December 31.
