BOSTON (Reuters) - In the wake of the stock market's May 6 chaos, more than 10,000 trades have been canceled, according to the Nasdaq OMX Group NDAQ.O.

Exchanges including Nasdaq and NYSE Euronext's NYX.N New York Stock Exchange agreed to cancel "clearly erroneous" trades after hundreds of stocks and exchange-traded funds lost as much as 99 percent of their value and then fully recovered in a 20-minute period on Thursday.

Regulators are still struggling to understand what caused the bizarre trading.

The Nasdaq “broke” 10,468 trades totaling 1.4 million shares in 236 different securities, Executive Vice President Eric Noll said on Tuesday in prepared testimony at a hearing of the House Committee on Financial Services in Washington, D.C. Noll did not give a dollar figure for the canceled trades.

Only trades that occurred between 2:40 p.m. and 3 p.m. EDT at prices at least 60 percent above or below a security’s price at 2:40 p.m. were canceled, prompting howls of protest from some investors.

Many complaining investors suffered considerable losses, but below the 60 percent threshold, when the inexplicable plunge triggered long-standing stop-loss sell orders. Others who complained had winning trades unwound. They said they should not be punished for stepping in to support the market and help end the steep price slide.

The controversial 60 percent level for revoking trades was set after “extended discussion” among exchange representatives after the market close on May 6, Noll said.

“There was significant debate among the exchanges regarding the proper break point for trades,” he said in his prepared remarks.

According to one person on the call, Nasdaq officials wanted even fewer trades canceled -- only those 80 percent away from the 2:40 price -- while other participants wanted the revocation trigger set at 50 percent or less. Thus the 60 percent level was chosen as a compromise.

Larry Leibowitz, chief operating officer at NYSE Euronext, said the cancellations had caused substantial confusion and were “an unsatisfactory substitute” for market-wide circuit breakers.

The U.S. Securities and Exchange Commission is reviewing the exchanges’ policies for cancelling trades, Chairman Mary Schapiro said in her testimony. The process must be “fair for investors and consistently applied -- both in the context of a single event and across different events,” Schapiro said in her prepared testimony for the committee.

ETFs were particularly hard hit by the chaos for reasons that “are still unclear,” Schapiro said. More than 25 percent of all such funds lost at least half their value. One ETF sponsor told the SEC that 14 of its funds briefly traded for less than 15 cents a share.