A power plant under construction in central Connecticut exploded with earthquake force that shook homes across much of the state on Sunday as workers purged natural gas lines in preparation for the plant to open this year. At least five people were killed and more than two dozen were injured as a section of the plant collapsed and burned.

Witnesses said the explosion at the Kleen Energy Systems plant in Middletown, 15 miles south of Hartford, occurred at 11:17 a.m. in a thundering convulsion of flames and smoke seen for miles around and felt as far away as cities and towns on the shore of Long Island Sound, 30 miles away.

As towering plumes of dark smoke poured into a dazzling blue sky, scores of ambulances, fire engines, police cars and helicopters streamed to the scene on the west bank of the Connecticut River on the southern outskirts of the city, the home of Wesleyan University.

Fire and rescue teams from Middletown, Durham, Portland, Cromwell and other towns converged as crews fought the blaze into the afternoon. Aerial pictures showed a smoking, sprawling riverside site with buildings housing generators, fuel tanks and other power equipment, topped by two smokestacks. The explosion apparently occurred in the largest building, called the Power Block, which was destroyed.