SINTRA, Portugal—Top officials from the world's two largest central banks on Saturday offered differing views regarding the role they should play in pushing for government economic overhauls, highlighting the diverging paths of major economies and the challenges of achieving inflation objectives.

In a panel discussion concluding a European Central Bank conference in a resort town near Portugal's western coast, ECB President Mario Draghi defended his stepped-up calls for economic reforms in Europe. He said this was one of the biggest issues for the region's economy, which has faced two recessions since 2009 and still has an unemployment rate of 11.3%—far above levels in the U.S. and Japan.

"In a monetary union, you can't afford having large and increasing structural divergences between countries," Mr. Draghi said. "They tend to become explosive. Therefore, they are going to threaten the existence of the monetary union."

The eurozone comprises 19 countries, each of which sets its own fiscal and economic policies while sharing a single monetary policy.

"We don't want to be intrusive" in urging governments to act on labor-market and fiscal-policy changes, Mr. Draghi said. "It's a policy appeal to action."

In contrast, Federal Reserve Vice Chairman Stanley Fischer urged a more cautious approach, though he noted the U.S. economy would benefit from greater infrastructure spending.

"You can talk about [structural reforms] from time to time, but you can't make this your main talking point every time you meet the press," he said during the panel discussion. "The temptation [to speak about structural reforms] is there, but we don't take it."

In part, this difference in emphasis between the Fed and ECB reflects the current states of their economies. In the U.S., where labor markets are considered more flexible and mobile than in Europe, unemployment was just 5.4% in April, far lower than in Europe.

"In Europe, the structural component of low growth is much bigger" than in the U.S., Mr. Draghi said, adding that tax and spending policies can play a more active role in boosting the region's growth prospects. "You can make a case for targeted, well-designed, growth-friendly fiscal expansion."

Speaking on the same panel, Bank of Japan Gov. Haruhiko Kuroda said monetary policy can make it easier for governments to introduce reforms to make their economies more flexible.

"If your economy grows with a 2% inflation rate, then even serious and severe structural reforms are maybe accepted by the general public," he said, adding that he expects inflation to hit the bank's 2% target in the first half of the 2016 fiscal year.

Japan's gross domestic product grew 2.4%, at an annualized rate, during the first quarter, the fastest of the three economies. Eurozone GDP expanded at a 1.6% annual rate, while the U.S. GDP grew just 0.2%. Still, the U.S. has had a more robust recovery since the global financial crisis on the back of large fiscal and monetary stimulus. The Fed is expected to start lifting interest rates from near zero later this year, while the ECB and Bank of Japan are purchasing large amounts of government bonds, a policy known as quantitative easing that the Fed also deployed aggressively in the years after the crisis but has since ended.

"Quantitative easing is unusual," Mr. Fischer said. "It looks like magic to many people. It looks like bad magic to a lot of people. Zero interest rates affect savers more than it affects people who are not yet at that stage of their lives," he said, adding that, ultimately, the economic benefits of easy-money policies outweigh that effect.

Mr. Draghi said he agreed that low interest rates had a "very, very significant" impact on savers in the eurozone.

"If QE will actually achieve its objectives of price stability or…lowering the unemployment rate, that's the best response to the critics" that say monetary policies have widened inequality, he said.

Write to Brian Blackstone at brian.blackstone@wsj.com

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