The Japan Meteorological Agency (JMA) has increased a volcano eruption warning for the southern island of Kyushu to the second-highest level.
The volcano is 50km from the recently restarted Sendai nuclear plant. JMA increased the alert level for the Sakurajima volcano to 4 from 3 on its scale of 5, advising people within 3km of the crater to prepare to evacuate after it detected multiple earthquakes in the area.
The area around the volcano had registered more than 800 tremors, according to the meteorological agency's website. Some 77 residents were advised to evacuate, the Fire and Disaster Management Agency said in a statement.
Kyushu Electric's Sendai continued to operate normally, a company spokeswoman said. Officials said big, burning rocks or pyroclastic flows could pose a danger to residential areas.
Authorities ordered the complete evacuation of Kuchinoerabu island off Kyushu in May. More than 50 people, mostly hikers, were killed in an eruption at Mount Ontake in Nagano in central Japan last September.
Agency officials say seismic activity increased rapidly on Saturday morning. They say they detected more than 1000 volcanic earthquakes through the day, including several that could be felt by people.
Volcanic activity has slowed since Saturday, but the agency remains on alert for signs of volcanic erruption.