Electrical power was slowly being restored in tsunami-ravaged northern Japan on Saturday after a strong aftershock, though more than a quarter-million homes remained in the dark.

A new wave of anxiety took hold as shoppers emptied store shelves and waited in long lines for gasoline after the magnitude-7.1 aftershock late Thursday.

It was another setback for those struggling to recover from the massive earthquake-spawned tsunami that wiped out hundreds of miles of the northeastern coast last month and killed as many as 25,000 people.

"I feel helpless. I am back to square one," said Ryoichi Kubo, 52, who was forced to shut down his gas station in hard-hit Iwate prefecture (state) when the aftershock knocked out power. He had just finally reopened after the power outage and prolonged fuel shortage that followed the March 11 tsunami.

Three people died in Thursday's aftershock, the worst since the day of the massive magnitude-9.0 quake. The latest tremor largely spared the nation's nuclear power plants, and there was no sign of fresh problems at the troubled Fukushima Dai-ichi complex, which has been spewing radiation since it was swamped by the tsunami.

Electricity remained out Saturday morning across parts of northern Japan, including areas far inland, and homes were without gas and water. Gasoline was again scarce.

Convenience stores sold out of basics such as water and snack foods, and supermarkets switched back to rationing purchases.

In Ichinoseki, 240 miles (390 kilometers) northeast of Tokyo, lines of 30 or more people formed outside the Marue supermarket when it opened at 9 a.m. Friday. With power out, each customer was escorted through the aisles by an employee with a flashlight and a pad, who jotted down the price of each item.

"I'm so tired. I just want to buy some chocolate," said Yuka Sato, 27, who patiently waited in line with her neighbors.

About 275,000 households were still without electricity Saturday morning. That includes homes in prefectures in Japan's northwest that had been spared in the March 11 quake.

Those killed by Thursday's aftershock were a 79-year-old man who died of shock, a woman in her 60s who lost power to her oxygen tank and an 85-year-old man whose cause of death was not specified.

A tsunami warning was issued, but it was canceled about 90 minutes later. The aftershock was in about the same location as the original 9.0-magnitude quake, off the eastern coast and about 40 miles (65 kilometers) from Sendai, an industrial city on the eastern coast. It was strong enough to shake buildings for about a minute as far away as Tokyo, about 200 miles (330 kilometers) away.

For tens of thousands of people living in shelters because they lost their homes in the tsunami or were evacuated from the area near Fukushima Dai-ichi because of radiation concerns, or both, the aftershock was an unpleasant reminder of what they have been through.

Matsuko Ito said she screamed when the violent shaking woke her up around 11:30 p.m. She's not sure she can take much more.

"It's enough," the 64-year-old said while smoking a cigarette outside the shelter where she has been living in the small northeastern city of Natori. "Something has changed. The world feels strange now. Even the way the clouds move isn't right."

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Associated Press writers Shino Yuasa, Malcolm Foster, Ryan Nakashima and Mari Yamaguchi in Tokyo and Eric Talmadge in Minami Soma contributed to this report.

A big aftershock rocked quake-weary Japan late Thursday, rattling nerves as it knocked out power to the northern part of the country and prompted tsunami warnings that were later canceled.

The quake was initially measured at magnitude-7.4, though the U.S. Geological Survey in Golden, Colorado, later downgraded it to 7.1. Either way, it was the strongest aftershock since several were recorded on March 11 — the day of the magnitude-9.0 earthquake and tsunami that killed as many as 25,000 people and touched off a nuclear crisis last month.

There were no immediate reports of serious injuries or major damage, and the operator of the tsunami-ravaged Fukushima Dai-ichi nuclear plant said there was no sign the aftershock had caused new problems there. Workers retreated to a quake-resistant shelter in the complex, with no injuries.

The aftershock around 11:30 p.m. was strong enough to knock items off store shelves and move a large automated teller machine at a FamilyMart convenience store in the major northern industrial center of Sendai. The city is far enough inland that it avoided major tsunami damage, but people there were without gas and electricity for weeks.

Manager Takehiko Akagi said 100 people had showed up within an hour of Thursday's aftershock and cleared the shelves of ice, water and instant noodles — items that were in short supply after the bigger quake.

"Usually at this time of night, there is almost no one," said Akagi, whose store had power even though people in nearby neighborhoods did not. A handful of buildings had broken windows and tiles, and some small electrical fires were reported.

In Ichinoseki, which is also inland, buildings shook violently, knocking items from shelves and toppling furniture, but there also appear to be no major damage there. Hotel workers lit candles so guests could find their way around.

Japan's nuclear safety agency said nuclear facilities along the northeastern coast were under control after backup generators kicked in at two — Rokkasho and Higashidori — that lost power.

The aftershock knocked out two of three power lines at the Onagawa nuclear power plant north of Sendai, which has been shut down since the tsunami. One remaining line was supplying power to the plant and radiation monitoring devices detected no abnormalities. The plant's spent fuel pools briefly lost cooling capacity, but it resumed because a power line was available for electricity.

"It's the way it's supposed to work if power is lost for any reason," said David Lochbaum, director of the nuclear safety project for the U.S.-based Union of Concerned Scientists. Since the tsunami warning was canceled 90 minutes after it was issued, there was no reason to believe the facilities' diesel generators would fail like the ones at the stricken Fukushima plant. The massive wave knocked out cooling systems and triggered a series of mishaps that have left workers struggling to stop radioactivity from spewing nearly a month later.

"That was really the blow that the plant didn't recover from," Lochbaum said.

Officials said the aftershock hit 25 miles (40 kilometers) under the water and off the coast of Miyagi prefecture. The USGS later downgraded it to 7.1.

Buildings as far away as Tokyo, which was about 200 miles (330 kilometers) from the epicenter, shook for about a minute.

Prime Minister Naoto Kan met with staff members in his office shortly afterward, according to deputy Cabinet spokesman Noriyuki Shikata.

A separate government emergency response team met shortly after midnight to monitor any reports of damage and urged firefighters, police and other emergency personnel to aid those in need.

USGS geophysicist Paul Caruso said the aftershock struck at about the same location and depth as last month's quake.

The USGS said the aftershock struck off the eastern coast 40 miles (65 kilometers) from Sendai and 70 miles (115 kilometers) from Fukushima.

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Associated Press writers Malcolm Foster, Ryan Nakashima, Mari Yamaguchi and Cara Rubinsky in Tokyo and Colleen Slevin in Denver contributed to this report.