That could help Rome and Madrid fend off market trouble until a strengthened eurozone bailout fund is approved to help them.
The bank did not say which countries' bonds it would buy in a statement after a crisis conference call this evening.
But the beneficiaries are expected to be Italy and Spain, market analysts say.
Italy and Spain are trying to avoid financial collapses like those that have forced Greece, Ireland and Portugal to seek bailout loans.
In a statement this evening, the ECB said it welcomed announcements by Spain and Italy on new fiscal and structural policy measures, and it urged both governments to roll them out swiftly.
‘This programme has been designed to help restore a better transmission of our monetary policy decisions - taking account of dysfunctional market segments - and therefore to ensure price stability in the euro area.
Earlier this evening France and Germany said they want full implementation of measures agreed at a eurozone summit in July in order to safeguard the single currency as markets brace for fresh turmoil this week.
‘President (Nicolas) Sarkozy and Chancellor (Angela) Merkel reiterate their commitment to fully implement the decisions taken by the heads of state and government of the euro area and the EU institutions on 21 July,’ a joint statement said.
Officials around the world are scrambling to head off fresh market turmoil on Monday as investors take on board Friday's unprecedented US ratings downgrade by Standard and Poor's on the grounds US politicians have failed to properly tackle the US debt problem.
