[Greek PM Tsipras looks on during a parliamentary session before a budget vote in Athens]Greek labour unions will stage a 24-hour nationwide strike against the leftist-led government's planned pension reforms demanded by international lenders, a union official told Reuters on Friday. The walkout against planned cuts in future benefits and increased social security contributions to make the pension system viable, will be a test for Prime Minister Alexis Tsipras's coalition which has a thin parliamentary majority. Participation in the strike, organised by Greece's largest private sector labour union GSEE and its public sector counterpart ADEDY, which together represent about 2 million workers and pensioners, will be crucial.