In foreign currency trading, the euro sank against the dollar to US$1.1401, its weakest level since early November 2003, around 1700 GMT following the ECB announcement compared with US$1.1620 beforehand.
The program of sovereign bond purchases comes after eurozone inflation turned negative in December, stoking fears that the 19-nation eurozone is on the brink of a dangerous deflationary spiral of falling prices.
"Mario Draghi has been left with little choice than to begin a more robust than expected quantitative easing program in a bid to awake the economies of the eurozone from their slumber," said Dennis de Jong, boss of trading site UFX.com.
"This play is seen by many as the last roll of the dice for the beleaguered euro. QE has had some success in the US and UK, but with such a patchwork of economies and banking systems in the eurozone, the jury is very much out.
"There will be a lot of people holding their breath over the coming months," he said.
The ECB chief also kept the door wide open to further purchases.
"They are intended to be carried out until end-September 2016 and will in any case be conducted until we see a sustained adjustment in the path of inflation," Draghi told a news conference.
The purchases, due to start in March, will thus total slightly more than 1 trillion euros.
"Given the importance of market effects as a key aspect of QE, the initial market reaction could be a strong steer to its ultimate impact," said Jonathan Loynes, chief European economist at research consultancy Capital Economics ahead of the decision.
Meanwhile, eurozone government bond yields fell on the prospect of massive ECB purchases.
The rate of return to investors on benchmark Spanish 10-year bonds hit a record low of 1.397 per cent before closing at 1.405 per cent, down from 1.530 per cent the night before.
The yield on Italian 10-year bonds was at 1.549 per cent near its record low of 1.548 per cent, down from 1.691 per cent on Wednesday.
French 10-year bond yields ended the day at 0.617 per cent after touching a low of 0.587 per cent, compared with 0.704 per cent the day before.
The ECB's strong move also gave a boost to shares on Wall Street.
The Dow Jones Industrial Average added 0.62 per cent to 17,662.29 points in mid-day trading in New York.
The broad-based S&P 500 rose 0.61 per cent to 2,044.58, while the tech-rich Nasdaq Composite gained 0.83 per cent at 4,706.38.
Before the ECB announcement, Asian markets had extended their rally this week, with Tokyo adding 0.28 per cent, Sydney rising 0.49 per cent, and ending the day Seoul flat.
Hong Kong rose 0.70 per cent and Shanghai gained 0.59 per cent.
