The quake had an epicentre about 22 km north-west of Athens, the BBC reported on Friday.
Athen ran out into the streets as the city shook for 15 seconds.
There were no reports of serious injuries, but several aftershocks have been felt and two buildings have collapsed.
The strongest aftershock had a magnitude of 4.3, almost an hour after the initial quake hit at 2.13 p.m. (Friday noon). Seismologists warned there could be more.
The earthquake was felt particularly strongly in the centre of Athens, where people stood in the streets after evacuating from tall buildings.
Plaster fell off the walls of the chief prosecutor's office in Athens and cracks appeared in the 170-year-old parliament building.
There were reports that the fire service, which has received 76 calls according to local media, rescued over a dozen people trapped in elevators following the power outage.
A pregnant tourist visiting the Archaeological Museum was hospitalised after she was accidentally struck by another tourist's elbow as they rushed for the exit.
Reports suggest at least three people were hurt by falling plaster.
An old residence and an empty building have collapsed, Greek media said. Other buildings were also damaged and pieces of marble have fallen from the Agia Irini church into the road.
Efthymios Lekkas, head of the earthquake protection agency, told people to remain calm.
"There is no reason for concern. The capital's buildings are built to withstand a much stronger earthquake," he said.
The quake was also felt in the southern region of Peloponnese.
Seismologists say the earthquake was around 13 km from the surface.
The earthquake is the first to hit the Greek capital since September 1999 and experts say it was very close to the same epicentre, at Mt Parnitha.
That magnitude six quake left 143 people dead and tens of thousands of buildings damaged.
( With inputs from IANS )