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By Daren Butler and Parisa Hafezi ISTANBUL/ANKARA (Reuters) - Protesters clashed with police in cities across Turkey on Tuesday after the death of a 15-year-old boy who was hit in the head by a tear-gas canister during anti-government demonstrations last summer. Police unleashed water cannon and tear gas on thousands of demonstrators, another pre-election headache for Prime Minister Tayyip Erdogan as he battles a corruption scandal that has become one of the biggest challenges of his decade in power. Istanbul and Ankara have both seen protests in recent weeks against what demonstrators regard as Erdogan's authoritarian reaction to the graft affair, which has included new laws tightening Internet controls and handing government greater influence over the appointment of judges and prosecutors. Berkin Elvan, then 14, got caught up in street battles in Istanbul between police and protesters on June 16 while going to buy bread for his family.