* Boy dies after 9 months in a coma
* Thousands protest in Ankara and Istanbul
* Calls on social media for further demonstrations
* Labour unions plan to attend funeral on Wednesday (Adds coffin carried through crowd, unions to join funeral)
By Mehmet Emin Caliskan
ISTANBUL, March 11 (Reuters) - Protesters clashed with police in Turkey's two biggest cities on Tuesday following 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 fired tear gas to disperse crowds in Istanbul and Ankara and more protests were expected later in the day, the latest headache for Prime Minister Tayyip Erdogan, already battling a graft scandal which has become one of the biggest challenges of his decade in power, weeks ahead of elections.
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.
Police fired tear gas and water cannon in the capital Ankara to disperse more than 2,000 people, mainly students from the Middle East Technical University (ODTU), who blocked a main highway to protest against the teenager's death.
Police also used tear gas in Istanbul after a crowd outside the hospital where Elvan died began pelting a police minibus with objects. Local media said at least 10 people were arrested.
Posts on social media called for protests later on Tuesday at dozens of meeting places around the country, while a few thousand people filled the central Kizilay square in Ankara, among them youths masking their faces with scarves.
Istanbul and Ankara have both seen protests in recent weeks against what demonstrators see as Erdogan's authoritarian reaction to the graft scandal, which has included new laws tightening Internet controls and handing government greater influence over the appointment of judges and prosecutors.
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 among the crowd.
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.
Local media said there were protests in the Aegean city of Izmir, where 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.
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 ruling centre-right AK Party ahead of the local elections on March 30 and a presidential race five months later.
Erdogan has slammed the illegal tapping of what should have been encrypted telephone conversations by Gulen's followers 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 in Istanbul and Mert Ozkan, Umit Bektas and Humeyra Pamuk in Ankara; Writing by Nick Tattersall; Editing by Gareth Jones)