Police cordoned off the area, but did not immediately approach the actual site of the explosion.
As dawn broke, television footage from the area showed the remnants of a car in the middle of an Athens street, with debris strewn around. It was unclear whether nearby buildings were damaged.
The news site that received one of the anonymous calls said the caller warned a bomb containing 75 kilograms (150 pounds) of explosives had been planted in a car and would explode in 45 minutes' time.