The explosion outside the Afghan defence ministry in Kabul has caused multiple casualties, Afghan officials have said.
A police spokesman said an apparent suicide attacker on a bicycle hit the main entrance to the building around 9:00am local time.
US defence officials have confirmed defence secretary Chuck Hagel, who is visiting the country, was in a safe location and unharmed.
He had been in a meeting at a coalition facility in the city at the time of the blast.
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The annual event has earned a place in the record books ahead of the grand final of the 60th anniversary show in Austria this evening.
Local authorities said the move is necessary to "avert the growing number of deaths and increasing harm linked to psychoactive substances".
Some students argued that the centuries-old tradition of wearing gowns, suits and mortarboards to exams was 'archaic' and looked elitist.