The jarring flash crash on May 6 yielded a record trading day at TD Ameritrade Holding Corp , though some of its customers were upset with the market breakdown that day, the U.S. online brokerage's chief said on Thursday.

Like everyone else, that day was a big day for us. We had a lot of trades, CEO Fred Tomczyk said at the Sandler O'Neill conference here. We don't talk too publicly about the number but that day was a record day for us.

With Europe's debt woes already weighting on the Dow Jones industrial average May 6, it plunged that afternoon some 700 more points in minutes before sharply rebounding, rattling investors globally and touching off a series of regulatory changes in the equities markets.

U.S. stock trading volume that day totaled about 19.18 billion shares, the year's biggest day by far, and the highest-volume day since October, 2008.

Not every investor or trader was happy with their execution because of what happened in the markets and market structure, Tomczyk said. But our technology held up very, very well that day. We did not have any outages.

Exchanges and regulators decided after markets closed May 6 to cancel trades that took place during a 20-minute window at prices well off what was considered reasonable for the specific shares.

A TD Ameritrade spokeswoman said a very small number of customer trades were adjusted after the crash -- less than a half a percent of all the trades processed that day -- but did not provide a specific number.

Kim Hillyer added in an email the company dealt with client issues on an individualized basis.

TD Ameritrade is the second largest U.S. discount broker and operator of the largest retail trading platform. The company and its peers are expected to report May trading volume, known as daily average revenue trades, or DARTS, later this month.

The sharp volatility last month is expected to give the industry a big boost.

(Reporting by Jonathan Spicer, additional reporting by David Gaffen and Edward Krudy; Editing by Bernard Orr)