Demonstrators have clashed with police on the streets of Athens and Madrid in an upsurge of popular anger at new austerity measures being imposed on two of the euro zone's most vulnerable economies.

In some of the most violent confrontations, Greek police fired teargas at hooded rioters hurling petrol bombs as thousands joined the country's biggest protest in more than a year.

The unrest erupted after nearly 70,000 people marched to the Greek parliament.

They chanted "EU, IMF Out!" on the day of a general strike against further cuts demanded by foreign lenders.

"We can't take it anymore - we are bleeding. We can't raise our children like this," said Dina Kokou, a 54-year-old teacher and mother of four who lives on €1,000 a month.

In Madrid, Prime Minister Mariano Rajoy faced violence on the streets of the capital yesterday, and growing talk of secession in Catalonia as he moves cautiously closer to asking Europe for a bailout, aware that such an action has cost other European leaders their jobs.

In public, Rajoy has been resisting calls to move quickly to request assistance, but behind the scenes he is putting together the pieces to meet the stringent conditions that will accompany rescue funds.

Rajoy presents a tough 2013 budget tomorrow, aiming to send a message that Spain is doing its deficit-cutting homework despite a recession and 25 percent unemployment.

Spain appears on course to miss its public deficit target of 6.3% of gross domestic product this year, and the central bank said the economy continued to contract sharply in the third quarter.

Rajoy is facing intense pressure from euro zone policymakers to take tougher measures, particularly on freezing pensions.

On Friday, Moody's will publish its latest review of Spain's credit rating, possibly downgrading the country's debt to junk status.

On the same day, an independent audit of Spain's banks will reveal how much money Madrid will need from a €100bn aid package that Europe has already approved for the banks.

Today's action in Greece was the first general strike since the three-party coalition government took office in June after two inconclusive elections.

Services across the country were disrupted with flights and trains suspended, and shops closed.

Despoina Spanou, an ADEDY civil servant union board member, said workers have already suffered at least a 50% reduction in salaries and said the union opposes additional austerity on a society that has already suffered.