At the ECB's last monetary policy meeting of the year, the governing council decided that the key deposit rate would be lowered to minus 0.30 per cent.
The deposit rate is normally the interest banks would receive from parking cash overnight at the ECB. But it has been negative since June 2014, meaning banks effectively pay the ECB to hold their funds.
The idea is to encourage banks to lend the money to businesses and households rather than store it at the central bank.
In a bid to correct this, the ECB decided to extend the purchases to March 2017 and possibly beyond and to widen the net to include other categories of bonds, Draghi said.
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Draghi insisted that the measures were working and that was why the ECB had decided to step up the QE programme.
The ECB was "doing more because it works, not because it fails," he said.
At the same time, "we had to recalibrate our measures due to changing circumstances over the summer," Draghi admitted, insisting that the ECB was ready to do it again if the "external conditions put at risk achievement of our objective."
In afternoon trade, Frankfurt's DAX 30 plunged 3.57 per cent, London's FTSE 100 index lost 1.06 per cent, and the CAC 40 in Paris dropped 1.89 per cent.
The euro meanwhile recovered from close to an eight-month low, jumping to nearly USD 1.09 before settling at USD 1.08 after having struck USD 1.0551 yesterday - which was the lowest level since mid-April.
"Santa Mario did not turn into the Grinch, the Christmas monster. However, (he) left many market participants disappointed like small kids who receive less and smaller presents than expected on Christmas eve," said ING DiBa economist Carsten Brzeski.
The reduction in the deposit rate was "disappointingly small," he said. And the ECB had failed to make up for that with a "decisive" expansion of its asset purchase programme, which had been "merely extended ... From September 2016 to March 2017.