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UPDATE 1-TD Ameritrade clients locked out for 80 minutes

* Online clients unable to log in 11:40 a.m.-1 p.m.

* Call wait times ‘elevated’ - spokeswoman

* U.S. markets dropped heavily on Thursday (Recasts with resolution, lockout details)

NEW YORK, May 20 (Reuters) - TD Ameritrade Holding Corp AMTD.O clients were unable to log into their online trading accounts for about 80 minutes as markets fell sharply on Thursday, the big U.S. online brokerage said.

The company was still investigating what went wrong between about 11:40 a.m. ET and 1 p.m., when individual clients were forced to contact company brokers by telephone to place orders, a spokeswoman said. Call wait times were “elevated,” she added.

U.S. stocks dropped sharply in heavy trading on Thursday, driving major indexes down more than 2 percent as Europe’s debt crisis rattled U.S. investors.

TD Ameritrade, which runs the largest U.S. online platform catering to individual or “retail” traders, said most clients who were logged on prior to the glitch were unaffected. Nor were wireless and downloadable platforms hit.

“We are also working with clients impacted by the issue individually to make sure that they are treated appropriately,” spokeswoman Kim Hillyer said. (Reporting by Jonathan Spicer; Editing by Gerald E. McCormick and Richard Chang)

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