HONG KONG (Dow Jones) -- Tokyo' s main electric utility may sue construction companies whose equipment knocked out power lines Monday morning and caused a blackout that affected more than a million homes and companies, according to reports.

Tokyo Electric Power Co. (TKECF) said the outage was caused when a construction crane being transported downriver on a ship operated by general contractor Obayashi Corp.hit a high-voltage power line.

The power company said unlisted construction firms Aoki Gumi and Mikuniya Kensetsu, which were also allegedly involved in the accident, may be mentioned in the lawsuit.

It did not indicate how much compensation it would demand.

Power was restored to most of the Tokyo area about two hours after the blackout struck.

The outages, which stranded commuters, trapped people in elevators and shut down computer systems at many firms including banks and brokerages, struck at about 7:40 a.m. local time.

The Tokyo Stock Exchange opened on schedule at 9 a.m. and trading did not appear affected by the blackout. The Nikkei 225 Averagewas up 1.45% in afternoon trading.

The opening of the government bond market was delayed slightly by the power outage. (END) Dow Jones Newswires 08-14-06 0136ET Copyright (c) 2006 Dow Jones & Company, Inc.