Repair crews yesterday completed restoring power to a six-block area of downtown Baltimore blacked out by a four-alarm fire that apparently was ignited by a cutting torch Monday night.

The blaze in some fiber glass molds started in the construction site for a station of the city's subway system, scheduled to begin moving passengers in 1983. A construction worker who fell 15 feet at the site was hospitalized with chest and leg injuries, and three firefighters were slightly injured.

The fire cut off electricity to 18 buildings in a six-block area, including police headquarters, the Hyatt Hotel and the National Aquarium. At the hotel a desk clerk reported that many of the approximately 500 guests appeared to be enjoying the blackout. "They stood around or went to the bar and said, 'Isn't this exciting?'" the clerk said.

A first priority at the year-old aquarium, a prime attraction of the city's inner-harbor development, became preservation of three chambered nautiluses, pearly-shelled mollusks housed in a small tank. Curators at the aquarium said the nautiluses are sensitive to temperature so they poured ice into the tank to keep the water below 60 degrees Fahrenheit as the Baltimore Gas and Electric Co. worked to restore power with the first available cable.

Electricity was restored to the aquarium at 11:58 p.m., about three hours after it went off it turned out, the denizens of the aquarium were not endangered.

The building has an emergency generator capable of keeping nautiluses and seals and jellyfish and sharks and just about anything else comfortable for as long as three days.