The blaze broke out just before 8 p.m. in an unoccupied pair of buildings under construction on East Main Street near Mountain Avenue in the commercial district.
Firefighters continued to douse hot spots into Monday morning.
Several streets were closed and some residents remained without power morning, prompting a two-hour school delay.
One firefighter suffered a sprained ankle but otherwise, no injuries were reported.
Neighbors said it was a matter of seconds before flames consumed the structures.
"This is a disastrous fire that is very difficult to contain and has the potential to spread to all nearby structures," Bound Brook police said.
The blaze could be seen several miles away and left nearly half of the town's PSE&G customers without power as of 10:30 p.m.
Service on NJ Transit's Raritan Valley Line was suspended between Bridgewater and Dunellen in both directions, with bus service as substitute transportation.
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