Tokyo (CNN) -- The most powerful earthquake to hit Japan in at least 100 years unleashed walls of water that swept across rice fields, engulfing towns, dragging houses onto highways and tossing cars and boats like toys.

Local media reported at least 32 deaths, with more casualties feared. And the 8.9-magnitude quake, which struck at 2:46 p.m., prompted the U.S. National Weather Service to issue a tsunami warning for at least 20 countries.

The quake's epicenter was 373 kilometers (231 miles) away from Tokyo, the United States Geological Survey said.

But residents there continued to feel aftershocks hours after the quake. More than 30 aftershocks followed, with the strongest measuring at 7.1.