“Longer-term investors should stay invested and not let this worry them. This is an opportunity to buy at lower prices,” Ms. Warne said.

Many investors are now growing nervous that central banks will raise interest rates in a bid to keep inflation at bay. If policymakers move too quickly, their efforts could temper the global growth.

Anthony Clemente, the chief executive of Canaras Capital, which specializes in high-yield loans and has $1.3 billion under management, said some of the market movement appeared to be an effort by portfolio managers to protect themselves from volatility in stocks rather than a broader fear of market turmoil.

“It’s really out of equities and inflows into the debt markets,” he said. “There’s an expectations that yields — fixed income and debt instrument — are the place where you will want to be at some point.”

Whatever the cause, there is now little doubt that markets are jittery.

After being lulled into a sense of complacency by years of steadily rising stocks, even small worries can snowball into a bad day for stocks. The losses can then feed on themselves in a financial industry dominated by computerized trading systems, with the weakness in the United States spreading around the world.

“Asia is going to be the tail that gets wagged by the U.S. dog,” Timothy Moe, chief Asia Pacific strategist at Goldman Sachs, said on Friday.