The volatility in financial markets certainly has grabbed the attention of former Treasury Secretary Larry Summers.

The Harvard professor argued in a Washington Post piece Monday that the Federal Reserve should refrain from raising interest rates soon, and he told CNBC Tuesday that we're going through a dangerous period.

Stocks couldn’t avoid plumbing Monday’s depths in a chart phenomenon known as a retest, where the lowest levels of previous days starts to influence trader psychology. The S&P 500 has now lost 11 percent in five days, the worst stretch since August 2011, with a measure of market turbulence known as the VIX sitting at more than twice its average level for the past three years, Bloomberg reported.

After a day of wild swings, the S&P 500 lost 3.9 percent Monday. That capped a 7 percent two-day retreat, the most since December 2008, sending the index into its first correction since 2011. The Chicago Board Options Exchange Volatility Index slid 12 percent Tuesday to 36.02. The VIX surged as much as 90 percent Monday to touch the highest level since January 2009 before closing at a nearly four-year high, Bloomberg reported.

"I'm not prepared to predict that we're in the midst of a crisis," Summers told CNBC. "But certainly the risks feel greater now than they have at moments in the past, and I think the orientation of policy, which had been toward resisting overconfidence, now has to again shift toward providing confidence."

There's a possibility of "significant instability," he said. Summers is especially worried about the VIX. "With the VIX ... the only argument you really have [for the Fed to raise rates] is, well it's uncomfortable [for the federal funds rate] to be at zero." Until Monday many economists had expected the Fed to raise rates next month, but now everything is up in the air.

While stocks plummeted in recent days, bonds soared, as investors have flocked to Treasurys bonds as a safe haven.

The U.S. 10-year note yield climbed seven basis points, or 0.07 percentage point, to 2.07 percent as of 4:59 p.m. Tuesday New York time, according to Bloomberg Bond Trader data. The 2 percent security due August 2025 fell about 5/8, or $6.25 per $1,000 face amount, to 99 11/32. The 10-year yield has dropped from about 2.5 percent in late June.

"Bond bears have misread economic growth. They have misread inflation data," Bob Andres, chief investment officer of Andres Capital Management, told The Wall Street Journal. “I think the U.S. economy will be in a slow-growth mode for longer than most observers believe.”

GDP grew a mediocre 2.3 percent in the second quarter, and the Atlanta Fed's forecasting model puts third-quarter growth at just 1.3 percent. Meanwhile, the Fed's favored inflation gauge rose only 0.3 percent in the 12 months through June. That's far below the central bank's target of 2 percent.

Related Stories:

• Harvard's Larry Summers: Fed Must Avoid Making 'Dangerous' Mistake
• Larry Summers: Yuan Devaluation Raises Risk of Deflation in US