A car bomb exploded in front of the gates of the Afghan intelligence agency on Wednesday, Reuters witnesses said, near heavily barricaded government buildings and Western embassies.
Shopkeepers and passersby were injured in the blast, which took place at noon (0730 GMT), but it was not immediately clear if anyone had been killed.
Shattered glass and twisted metal lay scattered in front of the gates of the National Directorate of Security (NDS) and gunfire and sirens were heard.
NDS head Asadullah Khalid narrowly survived a suicide bomber's assassination attempt last month in a brazen attack that threatened to derail a nascent and already fragile peace process between the Afghan government and the Taliban.
When contacted by Reuters, Taliban spokesman Zabihullah Mujahid said the group was "unaware" of the car bomb.
Wednesday's attack came just days after President Hamid Karzai returned from a trip to Washington, where he discussed the country's future with U.S. President Barack Obama once most NATO-led troops withdraw as planned by the end of 2014.
On that trip, Karzai visited Khalid in a U.S. hospital, where he was recovering from the Kabul assassination attempt by a man who had hidden a bomb in his trousers.
Violence across the country has been increasing in recent months, sparking concern over how the 350,000-strong Afghan security forces will be able to manage once foreign troops withdraw.