All trades in American Electric Power Inc and NextEra Energy Inc in a crash that happened in the first minute of trading on Thursday will stand, the New York Stock Exchange said, following the latest in a flurry of unexplained sharp drops in the market.

The stocks each dropped more than 50 percent in the first minute of trading, though ended just down slightly.

The share drops were the latest such incidents for the market since the May 6, 2010, "flash crash," a computer-driven trading glitch that caused a sudden afternoon tumble in the major U.S. indexes. Last week, the NYSE canceled trades in Anadarko Petroleum Corp after a blip in trading cut the market value of the company by 99 percent.

The latest event generated criticism from NextEra and others over what it means for investors.

"This is naturally a concern for all our shareholders and potential shareholders," said Moray Dewhurst, vice chairman and chief financial officer of NextEra Energy. "This type of market behavior is not what we would expect from a well-functioning and well-regulated exchange."

The NYSE said while all transactions in those shares in the first minute of trading will stand, all trades in AEP at or below $46.03 in that period, and all trades in NextEra at or below $76.19, will be marked with an "aberrant report indicator."

An aberrant report indicator is used to indicate the market believes that the trade price does not accurately reflect the prevailing market for the security, according to the U.S. Securities and Exchange Commission.

Shares of AEP dropped as much as 54 percent while NextEra sank 62 percent, both hitting their lowest price of the day right at the opening, but ended the session with small losses.

The Standard & Poor's utility index , which contains both AEP and NextEra, led declines on the S&P 500. It ended down 0.8 percent, after having fallen as much as 8.1 percent earlier.

Individual stock moves of 10 percent or more in a five-minute period usually trigger a trading halt, based on SEC rules, but the rules do not apply to the first 15 minutes of trading or the last 30 minutes of a session. The exchanges are able to cancel trades in the event of irregular or erroneous activity.

To further reduce volatility, the SEC has approved a program called "limit up/limit down" on all major stock exchanges. But the program is just starting to be introduced on large-cap stocks.

Hundreds of trades occurred in AEP and NextEra shares in the first minute of trading, though many were on small volume, such as 100 or 200 shares. AEP's stock hit a low of $22.28 in the early action, but ended the day at $48.28, down just 0.6 percent. NextEra hit a low of $30.37 and closed at $78.22, down 1.2 percent.

After the NYSE ruling, AEP's intraday low stood at $46.07, while NextEra's low was at $76.50.

"Essentially you have a market that's unprotected for the first 15 minutes of the day and the last half hour," said Joe Saluzzi, co-manager of trading at Themis Trading in Chatham, New Jersey. He noted, though, that the newer rules should help prevent the problem from happening in the future.

The limit up/limit down program aims to halt the trading of U.S.-listed stocks if they moved outside a recently traded price range.

"Limit up/limit down was effective April 8, (but) there is a further rollout period by ticker in alphabetical order," Credit Suisse analysts wrote in a recent research note. "So while a stock may be part of the S&P 500, it may not enter the pilot until a few weeks into the rollout."

(Reporting by Caroline Valetkevitch, additional reporting by Angela Moon; editing by James Dalgleish, Jeffrey Benkoe, Marguerita Choy and Leslie Adler)