BUENOS AIRES (Reuters) -Argentina's largest union started a 12-hour strike on Wednesday with tens of thousands of workers demonstrating in the heart of Buenos Aires against tough economic austerity measures and reforms by new libertarian President Javier Milei. The action, hitting sectors from transport to banks, is the biggest show of opposition to Milei's plans for spending cuts and privatization since he took office last month, pledging to fix an economy reeling from 211% inflation and crippling debt. The strike was coordinated by the powerful umbrella union the General Confederation of Labor (CGT) and comes amid major scrutiny of Milei's two major reform pushes: an "omnibus" bill going through Congress and a "mega-decree" deregulating the economy.