The leaders of the 16 countries that use the single currency said on Friday after talks with the European Central Bank and the executive European Commission that they would take whatever steps were needed to protect the stability of the euro area.
Both Italian Prime Minister Silvio Berlusconi and French President Nicolas Sarkozy cancelled trips to Moscow to mark the anniversary of the end of World War II in order to continue consultations over the crisis, though German Chancellor Angela Merkel said she would still go.
Financial markets have been pounding euro zone countries with high deficits or debts as well as low economic growth, threatening to force Portugal, Spain and Ireland into a position where, like Greece, they would need to seek financial aid.
The euro zone leaders, who have been accused of heightening market uncertainty with a lack of action, agreed to accelerate budget cuts and ensure deficit targets are met this year.
But they also decided, under pressure from the markets, to ask all 27 EU countries to agree a financial mechanism to ring-fence the Greek crisis before markets open on Monday.
"The euro zone is going through the worst crisis since its creation," Sarkozy said after Friday's euro zone summit in Brussels.
"The leaders have decided to put in place a European intervention mechanism to preserve the stability of the euro zone. The decisions taken will have immediate application, from the point that financial markets open on Monday morning."
"If the domino effect begins, no economy is safe," Finnish Prime Minister Matti Vanhanen told the Finnish broadcaster YLE on Saturday.
Euro zone sources said late on Friday that the mechanism could be funded by bonds issued by the European Commission with guarantees from euro zone states.
No details have been disclosed so far, but the sources said EU law provided a legal basis for such a mechanism.
The treaty governing the EU says that if a member of the 27-nation bloc is in difficulties caused by circumstances beyond its control, EU ministers may grant it financial assistance.
"Two mechanisms have been agreed - one based on article 122.2 of the Treaty saying the council can help a member state with serious difficulties," one of the sources said.
"The other will enable the European Commission to go on the markets and get money with an explicit guarantee of the member states and an implicit guarantee of the ECB (European Central Bank," the source added.
A second source said: "The details of this mechanism will be agreed by Sunday and the idea is to trigger both on Sunday."
Friday's EU summit approved $110 billion euros ($147 billion) in emergency EU/IMF loans to Greece over three years to help it over a budget crisis in exchange for austerity measures so sharp that they have already sparked violent protest.
There was some sign that popular anger might be subsiding as a new survey indicated that more than half of Greeks would rather back the EU/IMF deal than risk bankruptcy by going it alone, and were willing to make more sacrifices.
But fears that the loans might not be enough to prevent a Greek default and avert a broader economic crisis kept world stocks near a three-month low, despite strong US jobs data. Group of Seven finance ministers discussed the situation in a conference call on Friday after U.S. Federal Reserve officials expressed concern, and agreed to monitor the markets.
Earlier on Friday, the German parliament approved its share of the rescue, the largest contribution by a euro zone country.
But five German academics filed a legal challenge, reflecting widespread German public opposition, arguing that the aid was not provided for under EU treaties, and would give rise to inflationary policies.
Germany's highest court on Saturday rejected their request to block the immediate release of a German loan.
Merkel, who initially resisted agreeing to Greek aid due to the opposition at home, told voters in a regional election that euro zone countries would "lead this fight for the stability of the euro together and with resolve".
She said this did not only mean financial discipline.
"Those who created the excesses on the markets will be asked to pay up - those are in part the banks, those are the hedge funds that must be regulated ... those are the short-sellers and we agreed yesterday to implement this more quickly in Europe."
