An underground explosion and flash fire early Friday knocked out power in a wide area and injured seven workers rerouting underground electrical equipment to make way for the downtown MetroRail project, officials said.

Three of the men hurt in the 3:30 a.m. explosion were critically injured.Windows in nearby buildings were shattered by the explosion, which also threw a 4,000-pound cast-iron manhole ring into the air.

The workers were in an underground vault rerouting and restoring underground electrical equipment that had been disconnected due to construction on the $3 billion subway project, DWP spokesman Ed Freudenburg said.

"A short somewhere caused a spark that in turn led to a flash fire," Freudenburg said.

The explosion knocked out power in a seven-block area, DWP spokeswoman Dorothy Jensen said. The busy Los Angeles Flower Mart, which supplies flowers to businesses and restaurants throughout the city, was also left without power, she said.