By Renee Maltezou and Angeliki Koutantou ATHENS (Reuters) - Greek Prime Minister Alexis Tsipras challenged hard-left rebels in his Syriza movement on Thursday to put their opposition to new bailout negotiations to an immediate membership ballot, as he seeks to assert his control over the ruling party. At a meeting of Syriza's 200-member central committee held in an old cinema hall with a giant red backdrop, Tsipras defended his decision to accept harsh bailout terms as the best deal anyone could win for Greece. Greece narrowly averted an exit from the euro zone for now when it struck an 11th-hour deal with lenders this month, but that cost Tsipras the support of a quarter of his lawmakers who accuse the party of betraying its anti-austerity roots.