* Greece, Germany say attack will not destroy relations
* Police collect 60 spent bullet cases from scene
* Germany is Greece's biggest creditor nation
* Resentment high after bailouts, austerity
* No claim of responsibility (Adds bullets found lodged in walls of residence, quote)
By Harry Papachristou and Karolina Tagaris
ATHENS, Dec 30 (Reuters) - Unidentified assailants opened fire on the German ambassador's residence in Athens with a Kalashnikov assault rifle on Monday in an attack seen as an attempt to sour relations between debt-laden Greece and its biggest creditor nation.
Police said about 60 shots were fired at the high-security residence on a busy street of a northern suburb. At least four bullets were lodged in the walls of the house and four hit the metal gate of its perimeter. No one was hurt.
Anti-German sentiment has grown during Greece's prolonged economic crisis and many of those struggling with record unemployment and falling living standards blame Germany's insistence on fiscal rigour for their economic woes.
Germany is the biggest single contributing nation to Greece's 240-billion-euro bailouts which have kept the country afloat since 2010 and saved it from bankruptcy. Germany has at least 15 billion euros ($20.67 billion) of bilateral loans extended to Greece as part of the bailout.
No one has claimed responsibility for the 3.40 a.m. (0140 GMT) attack which police believe was carried out by members of leftist guerrilla groups. A police official, who spoke on condition of anonymity, said at least two assailants on foot were involved.
"Whoever is responsible for this act: You will not succeed in disrupting the close and friendly relations of our two countries," said German Ambassador Wolfgang Dold, who was at home at the time of the incident.
The residence was the target of an attack in 1999 when members of the now dismantled extremist group November 17 fired a rocket-propelled grenade that hit its roof.
Pictures lampooning German Chancellor Angel Merkel are commonplace in Athens while groups opposing Greece's bailout frequently protest outside the German Embassy. Public sector workers pelted a German diplomat with water bottles and coffee in a protest over austerity measures last year.
Monday's attack drew condemnation from across the political spectrum, with the anti-bailout opposition Syriza party saying it undermined Greece's struggle against austerity.
"Who benefits from the attack?" asked Syriza lawmaker Manolis Glezos, a hero of Greek resistance to the Nazi occupation of World War Two. "Certainly not the Greek people".
"We condemn the attack but this doesn't erase our anger at Merkel's policy against our country," said Terence Quick, foreign policy spokesman of the Independent Greeks, another anti-bailout party.
Greek Prime Minister Antonis Samaras telephoned Merkel after the incident and Greek Foreign Minister Evangelos Venizelos said it was a "cowardly terrorist act" which targeted Greece's image.
With a reputation for being Europe's problem child, Greece takes over the European Union presidency for six months from Jan. 1, hoping to show how far it has come since it almost crashed out of the euro zone common currency bloc 18 months ago. (Additional reporting by Yannis Behrakis in Athens and Madeleine Chambers in Berlin; Editing by Janet Lawrence)