By Inmaculada Sanz MADRID (Reuters) - Hundreds of thousands of Spaniards rallied in Madrid on Saturday against poverty and EU-imposed austerity in a largely peaceful protest later marred by violent clashes in which police fired rubber bullets. Some protesters started to throw stones and bottles at the large numbers of riot police present and attacked cashpoints and hoardings. The police fired rubber bullets to disperse them, according to video footage seen by Reuters. Central government representative Cristina Cifuentes said 19 protesters had been arrested and 50 police officers had been injured, one of them very badly, in the clashes.