The blast is said to be heard from up to 20 miles away at about 11:19 a.m. at the Kleen Power Plant in Middletown, a suburb of Hartford, Connecticut.
"There is no threat to anybody from either substances getting into the atmosphere or of a possible subsequent explosion," Middletown Mayor Sebastian Giuliano said, adding that terrorism has been ruled out.
He said plant workers were purging a natural gas pipeline when the explosion occurred.
It remains unknown how many people were working in the plant, which was expected to go online this summer, at the time of the explosion, Giuliano said.
Emergency room physician Dr. Jonathan Bankoff told reporters that some patients reported being thrown 30 or 40 feet by the blast.
After the explosion, it took a while for emergency crews to get into the plant, Deputy Fire Marshal Al Santostefano said, because the plant was on fire and the natural gas had to be turned off at the source. No major incidents at the site had been reported since construction began there a couple of years ago, he said.
The site is a 620-megawatt gas-fired power plant, according to plant manager Gordon Holk.