KIEV, Ukraine — Several thousand supporters of opposition leader Mikheil Saakashvili rallied in the Ukrainian capital Sunday, demanding his release from jail and calling for the country’s president to be impeached. Saakashvili, the former president of Georgia turned opposition leader in Ukraine, was arrested Friday on allegations that he colluded with Ukrainian businessmen tied to Russia to topple President Petro Poroshenko. The opposition leader rejected the allegations, and his lawyer said he was refusing food to protest his detention.