A total of 60 billion ($NZ90.9 billion) will be purchased each month from March 2015 until September 2016, and beyond if needed.
All eyes were on ECB president Mario Draghi as he delivered a bigger stimulus package than expected.
Commentators said the announcement was a historic crossroads for European markets.
Westpac economist Imre Speizer said the quantitative easing programme would run for 18 months or until there was sustained evidence inflation expectations were rising and the inflation outlook was improving.
The ECB sought not to have seniority over other creditors by limiting its purchases to 25% of a particular issue and 33% of a particular issue.
Those limits were judged not to give the central bank minority blocking rights in any creditor decisions in the event of a default, he said.
The programme was implemented despite opposition from Germany's Bundesbank and concerns in Berlin it could allow spendthrift countries to slacken economic reforms.
By September next year, more than 1 trillion would have been created under the QE programme, the ECB's last remaining major policy option for reviving economic growth and warding off deflation.
The news impressed markets.
Mr Speizer said the euro fell sharply, the US dollar rose, interest rates fell and equities rose - all predictable reactions given the upside surprise.
Craigs Investment Partners broker Chris Timms said the move pushed United States stocks to record their best gains in two weeks.
The volatility index, one gauge of investor uncertainty, fell 14% to 16.16.
European stocks rose to more than a seven-year high after the ECB laid out its plan, but the euro fell to an 11-year low against the US dollar.
Record low euro zone interest rates had failed to boost the 19-country euro zone, he said.
The ECB said it would keep euro zone interest rates at 0.05%. Rates have been at that level since September last year.
Earlier this month, figures showed the euro zone was suffering deflation, creating the danger growth would stall as businesses and consumers shut their wallets as they waited for prices to fall, Mr Timms said.
How it works
• In March, the ECB and national central banks of euro zone member states will start buying 60 billion ($NZ90.9 billion) of chiefly government debt each month.
That figure includes rebundled private debt, asset-backed securities and covered bonds, typically worth about 10 billion, on top of the roughly 50 billion in state bonds.
• The plan is to buy until September 2016 or until there has been a ''sustained'' improvement in consumer price inflation, which recently turned negative. The programme could end earlier if successful, or be extended if its impact is small.
• A total of 12% of the buying will be in the secure debt of European institutions - the European Investment Bank as well as bodies set up to help troubled countries in the euro crisis, the European Stability Mechanism and the European Financial Stability Facility.
A further 8% of the overall purchases will be government bonds bought directly by the ECB.
Any risk will be shared across the entire euro zone.
• The remainder - 80% of the government bonds - will be the responsibility of national central banks.
They bear the risk.
• Bonds with a credit rating of BBB-, one notch above junk, qualify.
Below that, a country must be in an aid programme.
A maximum of 33% of the bonds issued by a country may be bought.
This means Greece would not qualify for now because the ECB and other euro zone central banks already own more than this amount.
Only 25% of any individual debt issue in circulation can be bought.
• Central banks that buy are on an equal footing with other bondholders in the event of default. Bonds with a maturity of between two and 30 years are in play.
