Kabul police spokesman Ferdows Faramarz said the incident has taken place near the U.S. Embassy and Ahmad Shah Massoud Square in the capital Kabul.
Immediately the type of the blast and the exact number of casualties were unknown.
Later on, the Interior Ministry said in a statement that it was a suicide bombing.
According to the statement, at least 22 civilians and six members of Afghan security forces were killed and 38 others including women and children were injured in the blast.
The Taliban militant group claimed responsibility of the attack.
Zabihullah Mujahid, a Taliban spokesman said in a statement that a suicide attacker of the group has targeted Afghan soldiers gathered near the gate of the personnel directorate of the Ministry of Defense in Kabul.