For now, the country has imposed austerity measures, such as cutting salaries and pensions and raising taxes to qualify for the package. The public reaction to the cuts, however, has been one of outrage and violence, with three people dying during riots in Athens on Wednesday.
Geithner, who has a "Bloomberg box" on his Blackberry -- providing real-time stock information -- was surprised to see what was going on. While the Greek debt crisis is creating a "backdrop of fear," one senior administration official told ABC News, this was altogether something else.
Obama too was very concerned. Within minutes Geithner and Larry Summers, director of the president's National Economic Council, had been summoned to the Oval Office to brief the president. They continued to do so throughout the day.
The Obama administration has long been concerned about the Greek debt crisis, officials say, specifically that the European Union has not acted quickly or boldly enough.
"The crisis in itself is manageable," an official says. "But they've taken their time."
Geithner will talk to his counterparts in Europe later today when he holds a conference call with G-7 nations. President Obama may talk to some European leaders later today as well.
Wall Street sources told ABC News Thursday that the possible error by Citi involved what was supposed to be a $16 million trade on an S&P 500 futures-linked contract. The trade was entered in billions instead, they said.
The trade is believed to have sparked a massive sell-off in shares of Procter & Gamble, because P&G is one of the single largest components of the S&P 500 average. The sell-off was enough briefly to trigger an automatic halt to trading in Procter & Gamble stock.
Citigroup denied the charge, saying the speculation is "unfounded."
"As we have said, based on our review, rumors about a trading error by Citi are unfounded," the company said in a statement today. "It is troubling that inaccurate and unfounded rumors were spread as far as they were."
Late Thursday, regulators from the Securities and Exchange Commission and the Commodities Futures Trading Commission said they would try to figure out why the market gyrated so wildly.
"The SEC and CFTC are working closely with the other financial regulators, as well as the exchanges, to review the unusual trading activity that took place briefly this afternoon," they said in a statement. "We are also working with the exchanges to take appropriate steps to protect investors pursuant to market rules."
Wall Street traders called it one of the scariest days ever, as they market take a nosedive in a matter of minutes.
"That was probably the most disturbing trading activity anyone is ever going to see in our lifetime," said Art Hogan, managing director of securities firm Jefferies.
Some traders said it was as if machines had taken over the New York Stock Exchange.
"It was terrifying," Bernard McSherry of Cuttone & Co. said on "GMA."
Greifeld said the situation wasn't just about one person hitting the wrong button, but a "confluence of factors" that caused panic selling.