The European Central Bank unveiled a major policy reversal Thursday, flagging plans for fresh measures to stimulate the eurozone’s faltering economy less than three months after phasing out a €2.6tn ($2.9tn) bond-buying programme, making it the first rich-country central bank to ease policy in response to a global slowdown.

The ECB said it would hold interest rates at their current levels at least through the end of this year — months longer than previously signaled — and unveiled a fresh batch of cheap long-term loans for...