FRANKFURT—European Central Bank President Christine Lagarde signaled that the bank will keep monetary policy loose for some time amid a resurgence in Covid-19 cases globally and signs of economic slowdown in China and the U.S. that have prompted caution from the Federal Reserve.

While the ECB said it would slightly scale back its massive bond-buying program to reflect brighter prospects for the eurozone economy, Ms. Lagarde emphasized at a news conference that the decision didn’t constitute a plan to reverse its easy-money policies.

That signals a likely divergence with the Fed, whose top officials have indicated in recent weeks that they are on track to begin scaling back, or tapering, their bond purchases later this year.

As global output returns to pre-pandemic levels and inflation surges around the world, major central banks are considering whether and how to phase out emergency stimulus programs. Those decisions, especially the Fed’s, are set to ripple through global financial markets at a time when the highly contagious Delta variant of Covid-19 is weighing on the economic outlook.

Analysts expect the ECB to hold its key interest rate below zero and keep buying bonds on a large scale for a long time to come—even after it phases out an emergency €1.85 trillion emergency program, equivalent to $2.2 trillion, which is due to end early next year.

The ECB said Thursday it would conduct bond purchases under its emergency program at a “moderately lower pace” over the next three months. Ms. Lagarde said the move simply reversed a decision made in March to step up bond buying to contain higher borrowing costs, a knock-on effect of the powerful U.S. recovery.

“The lady isn’t tapering’, Ms. Lagarde said, echoing a phrase used by Margaret Thatcher in 1980 to signal her determination to see through economic reforms.

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Analysts said the ECB would likely slow its purchases to around €60 billion to €70 billion of eurozone debt a month through the end of the year, down from around €80 billion a month at present—still enough to soak up eurozone governments’ expected bond issuance for the rest of the year as they splurge on support for businesses and workers.

In the U.S., Fed Chair Jerome Powell has indicated that the central bank is likely to reduce asset buying later this year. However, weaker job growth in August is likely to spoil the case for the Fed to start reducing its $120 billion in monthly bond purchases at its next policy meeting on Sept. 21-22.

Investors took the ECB’s move in stride. European stocks made modest gains, with the Euro Stoxx 50 index rising 0.7% since the monetary policy decision went out. European government bonds rallied, particularly those issued by Southern European countries. The euro seesawed against the dollar before settling close to the level it was at before the meeting.

“It wasn’t a very hawkish meeting. Lagarde went to length to explain they’re not in a position where they think everything is over. With the bond market being up and the euro being down, on balance the market is interpreting this as being on the dovish side,” said Peter Schaffrik, a global macro strategist at RBC Capital Markets.

The eurozone economy has expanded strongly in recent months, driven by higher household spending as social restrictions aimed at containing the virus were rolled back. Analysts see scope for further gains as households spend the large stocks of savings they accumulated during the pandemic.

Ms. Lagarde said she expects the eurozone economy to return to its pre-pandemic level by the end of the year, earlier than previously expected. “The services sector is benefiting from people returning to shops and restaurants and from the rebound in travel and tourism,” Ms. Lagarde said. “Manufacturing is performing strongly, even though production continues to be held back by shortages of materials and equipment.”

Inflation in the 19-nation eurozone rose to 3% in August, the highest level in almost a decade and above the ECB’s target of 2%. That compares with consumer price inflation of 5.4% in the U.S. in July.

Analysts expect the eurozone inflation rate to rise higher over the coming months before falling back next year, driven by energy prices and supply shortages. Some more hawkish ECB officials have started calling for a prompt end to the ECB’s emergency stimulus policies amid rising inflation.

Many analysts expect a decision to phase out the ECB’s emergency program in December, although the bank is expected to compensate by ramping up an older bond-purchase program.

While the spread of the Delta variant has so far not required lockdown measures to be reimposed in Europe, it could slow the recovery in global trade and the full reopening of the economy, Ms Lagarde warned. Meanwhile the region has more than two million fewer people employed than before the pandemic, especially among the younger and lower skilled, she said. The number of workers in job retention schemes also remains substantial.

The eurozone economy was still about 3% smaller than its precrisis level by the middle of this year, even as the U.S. and global economies had already returned to their pre-pandemic size, according to Oxford Economics.

Corrections & Amplifications
Many analysts expect the ECB to unveil plans to end its emergency bond purchases in December, lagging behind a similar expected move by the Fed in November. An earlier version of this article incorrectly said they expect the ECB to unveil plans to end its purchases in November, lagging behind an expected move by the Fed in December. (Corrected on Sept. 9)

The ECB’s move to scale back its stimulus, while limited, sits awkwardly with a new policy framework unveiled in July, under which the bank committed to keep its easy-money policies in place for longer. Investors are starting to worry that the ECB could again make a similar error to the one in 2011, when it raised interest rates in response to higher inflation just before a recession, said Krishna Guha, a strategist at Evercore ISI.

—Anna Hirtenstein in London contributed to this article.

Write to Tom Fairless at tom.fairless@wsj.com