A 6.8-magnitude earthquake has shaken south Japan, leaving at least 50 people injured, with some trapped and presumed dead inside a partly collapsed shopping mall.

The earthquake hit the Kumamoto prefecture on the southern Kyushu island late on Tuesday afternoon, knocking out power to thousands of homes and damaging roads, bridges and buildings.

Japanese prime minister Sanae Takaichi told reporters authorities were still assessing the extent of the damage in the area, which was devastated by another deadly quake a decade ago.

“We have already been informed that people have been injured. Power outages and fires have occurred in some areas, and there has also been damage to roads and bridges and the collapse of buildings,” Takaichi said.

“I ask everyone to take action to protect themselves, including evacuating to a safe location.”

About 300,000 people have been instructed to go to evacuation centres, the disaster management agency said.

The areas with the biggest tremors will remain at risk of further strong quakes and landslides for about a week since the shallow earthquake is triggering active seismic activity in the area, according to the Japan Meteorological Agency. So far there have been more than 60 aftershocks ranging in magnitude from 1 to 4, the agency said.

A tsunami warning was issued immediately after the quake, but was lifted within two hours.

One hospital reported more than 50 injuries, public broadcaster NHK said. Several people on a high-speed train at the time of the quake were also injured, according to local media reports.

An explosion occurred in an Aeon shopping mall shortly after the earthquake, trapping “many people”, fire services said. Broadcaster TBS reported that police said “quite a few” were presumed dead in the incident.

An Aeon spokesperson said customers and employees were evacuated after the initial quake and the cause of the ensuing explosion remained unclear.

Kazuya Tsurunaga told TBS he heard the explosion while cleaning up dishes broken by the earthquake inside a pub about 300 metres from the Aeon mall.

“It was a huge impact,” he said. “A large cloud of smoke billowed up, and because the wind happened to be blowing and the wind direction was towards the shop, the area around us was in a state like volcanic ash was falling.”

Footage from NHK showed several other buildings on fire or partly collapsed, along with large fissures appearing on major roads, including an elevated highway.

TSMC, the world’s largest contract chipmaker, Sony and Fujifilm evacuated workers from their factories in the area.

The quake’s epicentre was about 20 kilometres (12 miles) south of Kumamoto, central Kyushu island’s largest city, with a population of about 700,000. The US Geological Survey initially said the earthquake had a magnitude of 7.1, but revised it later to 6.8.

Japan, which is located along the “ring of fire” of volcanoes and oceanic trenches in the Pacific basin, is one of the world’s most earthquake-prone countries, with a tremor occurring at least every five minutes. The country accounts for about 20% of the world’s earthquakes of magnitude 6 or more.

A massive earthquake in Kumamoto 10 years ago killed 275 people and injured a further 2,739, according to an official count, and damaged thousands of buildings including the city’s castle, a top tourist site.

Some of the castle’s stone walls are said to have been damaged in the most recent quake.

Kyushu Electric Power said 48,000 homes had lost power, while railway company JR Kyushu suspended services, including its high-speed bullet trains. Kumamoto airport also shut its runway as airlines diverted and cancelled flights.

Telecoms operators KDDI and NTT Docomo said there was disruption to their mobile phone services due to power shortages and transmission-line failures.

There were no irregularities reported at the nuclear power stations in the area, Japan’s nuclear regulation authority said.