BELGRADE (Reuters) -Thousands gathered in the centre of Belgrade in an anti-government protest on Sunday to demand the annulment of parliamentary and local elections a week ago that international observers said were unfair. The populist ruling Serbian Progressive Party (SNS) won 46.72% of the votes in snap parliamentary elections last weekend, according to state election commission preliminary results. An international monitoring mission on Monday said the SNS gained an unfair advantage through media bias, the improper influence of President Aleksandar Vucic and voting irregularities such as v…