Media as well as local residents urged the nuclear industry to take heed of the threat and make sure reactors were safe.
"When you have something like this, it's scary," said retired taxi driver Tomiji Okura, 72, in Kashiwazaki, a city of about 95,000 whose economy relies heavily on the nuclear industry along with fishing. "I want them to be made safe."
Nearly 800 homes were destroyed or damaged in Niigata alone and much of the water, gas and electricity supplies cut by the quake had yet to be restored on Tuesday.
About 9,000 people were set to spend a second night in schools and other make-shift evacuation centres.
"I've barely slept," said 35-year-old Kazuko Uchiya, a piano teacher who was at an evacuation centre with her 6-year-old son.
"I don't know when I can go home," she said.
Nine elderly people were killed and one person was missing, a Niigata prefecture official said.
Worries were mounting about the health of evacuees, many of whom are elderly.
"The damage was worse than anticipated," Kashiwazaki Mayor Hiroshi Kaeda told reporters. "If we can restore water services more people can go home, so that's what we want to do first."
Streets in Kashiwazaki were lined with damaged or collapsed houses, mostly wooden structures with heavy tile roofs, and many roads were blocked because of cracks, causing traffic jams.
Some people worked on repairs, covering damaged roofs with blue plastic sheets, while others picked through scattered rubble and many lined up for fresh water, which was trucked in by local officials and about 500 members of the armed forces.
Helmeted soldiers in camouflage uniforms made rice balls to hand out at evacuation centres, where crowds huddled sitting on "tatami" straw mats with blankets and a few belongings.
Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes, and for many Niigata residents, there was a tragic sense of deja vu.
The prefecture was hit in October 2004 by a quake, also with a magnitude of 6.8, that killed 65 people and injured more than 3,000. It was the deadliest quake in Japan since a magnitude 7.3 tremor hit Kobe city in 1995, killing more than 6,400.
(By George Nishiyama, with additional reporting by Isabel Reynolds, Osamu Tsukimori, Chikafuji Hodo, Elaine Lies and Linda Sieg)
