Military helicopters swarmed the quake zone 250 miles north of Tokyo, ferrying in supplies and flying the injured to hospitals. Officials said at least 144 people were injured, and landslides trapped 100 bathers at a hot spring resort. Local media reports said the number of injuries exceeded 200.
At a nuclear power plant in Fukushima, the jolt splashed 5 gallons of radioactive water from two pools storing spent fuel, but there was no leakage outside the plant, said Trade and Industry Ministry official Yoshinori Moriyama.
The force of the quake, which was followed by some 153 aftershocks, buckled many roads, including one highway that was severed when a stretch of land collapsed, creating a cliffside. Electricity was cut to about 29,000 households, though service was mostly restored by Saturday night.
Prime Minister Yasuo Fukuda said the government was mobilizing troops, police and "everybody we possibly can" to find the missing and rescue and treat the injured. He ordered rescue workers to continue their efforts through the night.
"Our most important task is to save as many lives as possible, and we are doing the best we can," he said.
The six dead included a man who ran out of a building in fear and was hit by a passing truck, four people buried by landslides, and a construction worker hit by a falling rock at a dam, officials said.
The 8:43 a.m. quake was centered in the northern prefecture (state) of Iwate, and was located about 5 miles underground. It was felt as far away as Tokyo.
"It shook so violently that I couldn't stand still. I had to lean on the wall," said Masanori Oikawa, an Oshu city official who was at home near the epicenter when the quake struck. "When I rushed to the office, cabinets had been thrown onto the floor and things on the desks were scattered all over the place."
The quake also knocked down equipment and car parts at the Iwate factory of Kanoto Auto Works Ltd., a Toyota Motor Corp. subsidiary that assembles popular Corollas and other models, company spokeswoman Seiko Watanabe said. The company has not decided whether to resume production Monday.
A semiconductor subsidiary of Fujitsu Ltd. halted production "as a precaution" but there was no major damage to the building or equipment, company spokesman Yasuhiko Youdou said.
Rescuers said their top problem was the fractured road system, which stopped them from reaching isolated hamlets in the damage zone.
"We're getting growing reports of damage, but we can't even get out there to assess the situation with roads closed off because of landslides," said Norio Sato, a city official in one of the hardest-hit towns, Kurihara.
In that town, a landslide buried 15 construction workers. Twelve managed to dig themselves out, but two others were killed and one remained missing. Seven people at Komanoyu hot springs were also missing after a separate landslide hit the resort, said another city official, Katsuyuki Sato.
Nearly nine hours after the quake hit Kurihara, Marue Supermarket manager Kazuya Kikuchi was still cleaning up the damage.
"A lot of liquor bottles fell on the floor, and we are still cleaning up the mess," he said, adding that he was keeping the store open for customers who needed daily supplies. "(The quake) was really scary, and I'm worried because we continue to feel many aftershocks."
The Defense Ministry dispatched a dozen helicopters and patrol aircraft, and the disaster agency mobilized about 760 rescue workers for the rescue and relief operation. The government also sent a CH-47 helicopter carrying Disaster Minister Shinya Izumi to the region.
"The extent of damage to roads and bridges was much larger than I had expected," Izumi told reporters.
Local governors were asking for more troops.
Footage shot from media helicopters showed landslides on rural roads running along mountains separated by long stretches of rice fields. Videos aired by public broadcaster NHK also showed a bridge that collapsed. NHK said four people were seriously injured while riding in a bus on a bridge when the quake hit, but it was unclear whether it was the same one.
Videos from the closest large city, Sendai, showed the force of the quake shook surveillance cameras for 30 seconds. Still, Sendai appeared largely unscathed.
"So far we have not received any reports of damage or injuries. Everything is normal," Hideki Hara, a police official in Sendai, told the AP. "Phone lines, water and electricity are all working right now."
Japan is one of the most earthquake-prone areas in the world. The most recent major quake in Japan killed more than 6,400 people in the city of Kobe in January 1995.