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. He slipped into a coma and became a rallying point for government opponents, who held regular vigils at the hospital where he lay in intensive care.

On Tuesday evening, police fired water cannon and tear gas in Ankara's central Kizilay square to scatter several thousand protesters who chanted: "Government of Erdogan, government of corruption, resign resign". The police pursued the protesters into sidestreets where small clashes continued.

There was similar police intervention against thousands of protesters on both the European and Asian sides of Istanbul, among dozens of places across Turkey where posts on social media had called for protests on Tuesday evening.

In the Mediterranean city of Mersin, two women were injured when struck by a water cannon vehicle, one of them suffering a head wound, the Dogan news agency said. Four police were also reported injured in the clashes there.

Police detained 20 people as they skirmished with protesters trying to march to the offices of Erdogan's AK Party in the Black Sea city of Samsun, Dogan reported.

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In the southern city of Adana, protesters threw stones aimed fireworks at police lines as water cannon vehicles advanced against them, spraying water. Large numbers also protested in the western cities of Izmir and Eskisehir in the most extensive protests since last summer's unrest.

Residents in some Istanbul districts banged pots and pans with spoons from the windows of their apartment blocks, reviving a form of protest popular during the summer.

Crowds chanted "murderer Erdogan" and "the murderer state will be brought to account" as mourners carried Elvan's coffin, wrapped in red cloth and strewn with red carnations, to a "cemevi", an Alevi place of worship, in central Istanbul.

Alevis are a religious minority in mainly Sunni Muslim Turkey who espouse a liberal version of Islam and have often been at odds with Erdogan's Islamist-rooted government.

Among the throng of up to 1,000 people, some waved plain red flags, while shopkeepers in the Okmeydani district pulled down their shutters as a mark of respect. Elvan's mother, flanked by a group of women, stood crying at an open window.

"We have come here because of the murderer police. They will be held to account. Berkin Elvan's blood will not be left on the ground," said Ahmet Ekinci, one of those in the crowd.

TURBULENCE AHEAD

Elvan was the sixth person to die in violence during nationwide protests in late May and June over Erdogan's plans to bulldoze an Istanbul park. The protests turned into one of the biggest shows of public defiance of Erdogan's 11-year rule.

President Abdullah Gul, the first senior figure to publicly comment on Elvan's death, sent his condolences to the family.

In the Aegean city of Izmir, school children and university students staged sit-ins. The Confederation of Revolutionary Trade Unions (DISK) said it would join Elvan's funeral in Istanbul on Wednesday.

"Their children steal billions, our children are killed going to get bread," DISK said in a statement, referring to the corruption scandal which erupted on December 17 with the arrest of the sons of three cabinet ministers.

Erdogan has cast the corruption investigation as a plot to unseat him by U.S.-based Islamic cleric Fethullah Gulen, a former ally whose followers say they number in the millions and who wields covert influence in the police and judiciary.

Gulen said in a message of condolence to Elvan's family, published in the Zaman newspaper that is close to him, that Turkey was experiencing difficult days and that the "mind of the state has become overwhelmed by anger and hatred".

"This atmosphere of hatred is undermining society's need for love and peace and efforts to understand one another," he said. "Little 15-year old Berkin Elvan is the latest victim of this atmosphere."

Leaked voice recordings, many of them purportedly of Erdogan, have appeared on YouTube over the past two weeks, part of what the prime minister sees as a campaign to sully his center-right AK Party ahead of the local elections on March 30 and a presidential race five months later.

Erdogan has condemned the illegal tapping by Gulen's followers of what should have been encrypted telephone conversations and has described some of the leaked recordings as "fabricated montage". On Tuesday, he rounded on the cleric again and suggested there could be more leaks to come.

"I ask you, for God's sake, you listen to people's private phone calls, you use them for slander and blackmail, indeed you go further and spy on and record people in their bedrooms," he told an election rally in the southeastern city of Bitlis.

Gulen has repeatedly denied unleashing the corruption investigation.

(Additional reporting by Daren Butler and Dasha Afanasieva in Istanbul Mert Ozkan, Umit Bektas, in Ankara, Humeyra Pamuk in Adana; Writing by Nick Tattersall; Editing by Mark Heinrich)