"The combined monthly purchases of public and private sector securities will amount to €60 billion" Draghi told assembled journalists at a press conference in Frankfurt.
The bond-buying programme will continue "until we see a sustained adjustment in the path of inflation which is consistent with our aim of achieving inflation rates below, but close to, two percent", he added.
Spending €60 billion a month for that long would mean the bank injecting over €1.1 trillion into the region's sputtering economy.
The euro immediately dropped sharply against the US dollar on the news, to below $1.15 – its lowest since 2003 – in a sign that the programme was more aggressive than markets had been expecting.
The Swiss franc rose to 1.01126 euros, but slipped against the dollar after several days of advances.