The Securities and Exchange Commission held urgent discussions on Friday to calm investors' raw nerves a day after a plunge of nearly 1,000 points in the Dow Jones industrial average, during which some stocks lost more than 60 percent of their value.

The decline, which stemmed from growing concern about Europe's ability to contain the Greek debt crisis but was widely believed to have been exacerbated by at least one large erroneous trade, sparked outrage among investors and politicians.

Chairman Mary Schapiro canceled an appearance at an industry trade group breakfast to focus on Thursday's trading glitch, said a colleague who attended the event in her place.

She felt it was important to remain at the office and follow up on unusual trading activity that took place briefly yesterday afternoon, said Andrew Donohue, director of the market watchdog's Division of Investment Management.

The SEC said on Thursday it was working closely with the Commodities Futures Trading Commission to review the unusual trading, which at its deepest point wiped nearly $1 trillion off equity values.

An SEC spokesperson declined to elaborate on that statement on Friday.

U.S. Treasury Secretary Timothy Geithner on Thursday held a conference call with Federal Reserve Chairman Ben Bernanke as well as with regulators from the SEC and the CFTC, a source said.

And a U.S. House of Representatives panel has slated a hearing on the causes of the market swoon for next Tuesday, with its chairman, Rep. Paul Kanjorski, urging the SEC to investigate as well.

The Nasdaq Stock Market early on Friday widened its list of stocks that will see canceled trades, and the focus turned to derivatives and regulators.

Trades that took place during the worst of the meltdown will be canceled for more than 250 stocks, Nasdaq OMX Group Inc said, adding to the long list of busted transactions on NYSE Euronext's Arca, other exchanges and trading venues.

The unusual exchange-wide agreement to cancel trades in stocks -- that fell more than 60 percent for a 20-minute period Thursday -- raised questions for futures and options markets, where many contracts are based on underlying stocks and stock indexes.

CME Group Inc, the giant futures market, had no immediate comment on whether it would cancel or adjust trades. Two major futures trading firms said CME had given no notice that trades would be broken.

A source said it is likely options trades based on the equities trades that were canceled would be adjusted, not busted.

At least one high-frequency trading firm said it stopped trading during the worst of the selloff, raising questions about the reliability of the all-electronic market-makers that provide much liquidity in today's markets.

(Reporting by Ross Kerber; Editing by Derek Caney and Steve Orlofsky)