The Afghan presidential contender, Abdullah Abdullah, has claimed election victory despite preliminary results putting him in second place.
Dr Abdullah, who fell just short of an outright majority in the first round, was well behind in the 14 June run-off.
Election officials say early counts show that the former Finance Minister Ashraf Ghani won 56.44 percent of votes in the run-off, while Mr Abdullah had 43.56 percent, the BBC reports.
Addressing supporters in the capital, Kabul, Dr Abdullah repeated claims that the election was marred by fraud.
Earlier, United States President Barack Obama spoke Dr Abdullah to call for calm and dialogue and urge a review of the fraud charges in the election, Reuters reports.
Speaking at a White House briefing, spokesperson Josh Earnest said serious allegations of fraud have been raised, but they have yet to be adequately investigated.
"We expect a thorough review of all reasonable allegations of fraud, that there is no justification for resorting to violent or extra-constitutional measures," "Any such move would cost Afghanistan the financial and security assistance of the United States."
US Secretary of State John Kerry has also warned against a power grab, amid reports that Dr Abdullah was planning a "parallel government".
Meanwhile, a bomb near Kabul has killed 16 people, including four Nato troops.
Ten civilians and two police officers were also reportedly killed in the attack on a clinic near Bagram, home to the largest US base in the country.