European leaders have reached a "three-pronged" agreement described as vital to solve the region's huge debt crisis.
They said banks holding Greek debt accepted a 50% loss, the eurozone bailout fund will be boosted and banks will have to raise more capital.
Shares on European markets rose sharply on news of the deal.
The agreement is aimed at preventing the crisis spreading to larger eurozone economies like Italy, but the leaders said work still needed to be done.
After marathon talks in Brussels, they agreed a mechanism to boost the eurozone's main bailout fund to about 1tn euros (£880bn; $1.4tn).
Banks must also raise more capital to protect them against losses resulting from any future government defaults.
BBC business editor Robert Peston says it is perfectly clear that EU leaders have bought some time, and for a few weeks and maybe longer the markets will give them the benefit of the doubt.
The framework for the new fund is to be put in place in November.
Meanwhile EU leaders welcomed Italian Prime Minister Silvio Berlusconi's pledge to balance his country's budgets and implement reforms to bring down its 1.9tn-euro debt.
Correspondents say Mr Berlusconi had been under huge pressure to prove he was serious about austerity measures.
The announcement of the deal helped lift the euro, with investors reacting positively to the outlook for the region's growth and single currency.
"The eurozone has adopted a credible and ambitious response to the debt crisis," a visibly tired French President Nicolas Sarkozy said at a news conference early in the morning in the Belgian capital.
"Europe is closer to resolving its financial and economic crisis," said Jose Manuel Barroso, president of the European Commission, in a report later to the European Parliament.
"We are showing that we can unite in the most difficult of times."
Fears about the state of the eurozone's finances and the threat of a break-up of the single European currency have been stalking markets for months.
Critics have accused policymakers of not doing enough to resolve the issues, contributing further to problems and fuelling uncertainty.
Leaders of the 17 eurozone nations had been in meetings since Wednesday trying to hammer out a deal to help Greece put its national finances in order and underpin other European economies such as Italy.
Speaking after the deal was agreed, Mr Sarkozy said that "the complexity of the files, the necessity to get everybody to agree, means that we have been negotiating for long hours".
He said he believed the result would be a relief for "the whole world", which had been expecting a strong decision from the summit.
Because banks have agreed to shoulder losses on Greek bonds, the country's burden has been reduced, cutting its debt down to 120% of its gross domestic product by 2020.
'Marathon not sprint'
Greek Prime Minister George Papandreou hailed the deal, saying: "We can claim that a new day has come for Greece, and not only for Greece but also for Europe."
The eurozone leaders also said the firepower of the main euro bailout fund - known as the European Financial Stability Facility (EFSF) - would be boosted from the current 440bn euros to about 1tn euros.
Bank recapitalisation - the third key element of the package - was agreed earlier.
The banks would now be required to raise about 106bn euros in new capital by June 2012, and governments may have to step in despite the unpopularity of further bank bail-outs.
It is hoped that this would help shield them against losses resulting from any government defaults and protect larger economies - like Italy and Spain - from the market turmoil.
"The package that we have agreed tonight, a comprehensive package, confirms that Europe will do what it takes to safeguard financial stability," said Mr Barroso, announcing the deal.
"I've said it before and I'll say it again, this is a marathon not a sprint."
German Chancellor Angela Merkel, who was a key negotiator in thrashing out the deal, said: "I think we were able to meet expectations and we have done what needed doing" for the euro.
IMF chief Christine Lagarde, who was also at the Brussels summit, said she was "encouraged by the substantial progress made on a number of fronts".