Authorities said at least 120,000 people were affected by the train delays, and the outage shut down 900 elevators in the capital region, including several dozen with people inside. Officials said most if not all the occupants were rescued soon afterward.
The crane that caused the blackout was perched on a barge traveling on the Kyu Edo River on the eastern edge of Tokyo when it hit power lines spanning the river, shorting them out, said Tokyo Power Electric official Kiyohito Yokoi.
The outage, which struck at 7:38 a.m., cut electricity to about 1.39 million households in Tokyo and the suburbs of Chiba and Kanagawa, up from the previously announced 800,000 households, according to Naoko Haruyama, another Tokyo Power official.
Public broadcaster NHK reported that construction workers aboard the ship did not realize that the 33-meter crane was raised too high.
The blackout also temporarily left Japan's largest business daily, the Nihon Keizai Shimbun, unable to update its Tokyo Stock Exchange Nikkei 225 index for about three hours in the afternoon.
Trading at the Tokyo Stock Exchange was carried out as usual after the central Tokyo bourse switched to emergency power sources.
The severed power lines are part of a massive grid serving the 35 million inhabitants of the Tokyo metropolitan region, which includes the cities of Yokohama and Chiba. One-fourth of Japan's population of 127 million lives there.
The last time Tokyo had a similar blackout was in November 1999, when electricity was cut to some 800,000 households after a Japanese military plane hit power lines, according to the AP.
The number of people affected by the transportation delays on Monday was limited by the four-day "bon" summer holiday, and the number of passengers during the morning rush hour was far fewer than usual.
Sony Corp. said the only part of its operations affected was a research and development division for cameras and other devices in southern Tokyo. The division continued operations using emergency electricity, the company said.