U.S. stock market indexes gained about half a percent in Friday's early trading.
In mid-day trading, London, Paris, and Frankfurt are down as much as two percent. Key Asian indexes were down sharply at the close of their trading day.
The focus on trading follows Thursday's wild session in New York that saw the Dow Jones Industrial Average drop nearly 1,000 points at one point. The Dow and other key indexes later recovered somewhat to post losses around three percent.
The Greek sovereign debt crisis played a role in Thursday's decline, but sources on the New York Stock Exchange say trading errors also may have contributed to the plunge.
European nations and the International Monetary Fund have put a $145 billion aid package together for Greece, in exchange for sharp cuts in salaries and public spending.
That austerity plan sparked riots in Athens, raising questions about whether the aid plan would be successful.
Some investors may have been disappointed that the European Central Bank ended a key meeting Thursday but did not outline any additional efforts to fight the debt crisis.
The Bank of Japan announced Friday it will offer $22 billion to financial institutions there, in an effort to boost liquidity.
Some information for this report was provided by Bloomberg.