Overhanging Yellen's speech was the possibility that it marks her final appearance in Jackson Hole as Fed chair. Her term as chair will end in February, and Trump has made clear he is considering replacing her. One candidate he has mentioned is Gary Cohn, a former Goldman Sachs senior executive who leads Trump's National Economic Council.

Some saw Yellen's forceful defense Friday of the regulatory structure imposed on banks since the 2008 crisis as further lessening the likelihood that Trump will reappoint her.

Sal Guatieri, senior economist at BMO Capital Markets, said Yellen's speech "puts her at odds with Trump's deregulation mandate, which could weigh against her remaining as chair when her term expires early next year."

Paul Ashworth, chief U.S. economist at Capital Economics, said that "Yellen's passionate defense of the post-crisis tightening of financial regulation isn't going to go down particularly well at the White House" and that it probably makes it less likely that Trump will re-nominate her.

At the same time, tensions might be arising between Cohn and Trump. In an interview published Friday in the Financial Times, Cohn said he considered quitting the White House over the president's widely condemned response to violence at a white nationalist rally in Charlottesville, Virginia. Cohn said he ultimately chose not to leave because of the duty he feels to his job.

A few hours after Yellen's speech Friday, Mario Draghi, president of the European Central Bank, told the Jackson Hole conference that the global economy is strengthening but warned that countries must work together to resist a growing backlash against open trade. He said multi-lateral cooperation is crucial to reassuring workers who worry that free trade puts their jobs at risk.

Draghi didn't address the financial health of the eurozone, which includes the 19 nations that share the euro currency. Some investors had expected him to send some signal about when the ECB will begin slowing its monthly bond purchases, which have been intended to keep borrowing rates low but are less needed now that the European economy is improving. The ECB is expected to take up the issue next month.

In her speech, Yellen noted that the U.S. and global financial systems were "in a dangerous place 10 years ago," with severe strains that led to the collapse of investment bank Lehman Brothers, the government takeover of mortgage giants Fannie Mae and Freddie Mac and the requirement that taxpayers bail out the largest banks.

The Fed chair pointed out that despite all the government support to banks, the downturn devolved into the Great Recession, with the loss of 9 million US jobs and millions of Americans losing homes to foreclosures. She said that the resulting recovery has been painfully slow and that policymakers have had to devise ways to prevent another financial crisis.

Yellen said the Dodd-Frank Act has produced a far safer banking system, especially with banks required to hold much higher levels of capital, the cushion against losses from bad loans.

"The United States, through coordinated regulatory action and legislation, moved very rapidly to begin to reform our financial system," Yellen said, "and the speed with which our banking system returned to health provides evidence of the effectiveness of that strategy." (AP)