* Thousands protest local election results
* Erdogan's AK Party dominated electoral map
* Opposition challenges counts in key cities (Adds clashes in southeast, details)
By Jonny Hogg and Gulsen Solaker
ANKARA, April 1 (Reuters) - Riot police fired water cannon in Ankara on Tuesday to disperse thousands of Turks demanding a partial recount in national polls that saw Prime Minister Tayyip Erdogan's ruling party sweep the electoral map.
The Islamist-rooted AK Party kept control of the two biggest cities, the financial centre Istanbul and the capital Ankara, and increased its share of the national vote in Sunday's municipal elections despite a corruption scandal dogging Erdogan's government.
The opposition said it would contest some of the results.
The crowd, calling for a recount of the Ankara result which was particularly close, gathered in front of the Supreme Electoral Council (YSK) chanting "Thief Tayyip!" and "YSK, the people are with you!" before the riot police moved in.
No official results have yet been announced, but the tally published by Turkish media put the AK Party on around 44 percent of the nationwide vote to 26-28 percent for the opposition CHP.
The election in NATO's only predominantly Muslim state took place amid a fierce power struggle between Erdogan and U.S.-based Islamic cleric Fethullah Gulen, whom he accuses of pursuing a dirty campaign of anonymous postings of audio recordings that implicate the prime minister in graft.
Erdogan denies corruption, just as Gulen denies any role in the recordings obtained from secret government communications.
Opposition supporters, many of them students who answered calls on social media, packed the basement of the main opposition CHP headquarters, working shifts through the night as they searched results sheets for signs of fraud.
"Whatever the election results are, it will unfortunately go down in the history of our democracy as a dubious election," the CHP's defeated mayoral candidate in Istanbul, Mustafa Sarigul, told a news conference.
"The theft of a single vote is a black mark for democracy."
Erdogan, flanked by his family, gave a victory speech to thousands of cheering supporters on Sunday from the balcony of the AKP headquarters as fireworks lit the midnight sky.
The result was a bitter disappointment for the CHP, despite the rise in its share of the national vote. The party failed to shrug off its image as a bastion of the secularist elite, aloof from the realities of life for the majority in this socially conservative nation of 77 million people.
The CHP is challenging the result in Ankara and in the southern coastal city of Antalya, traditionally a CHP stronghold that fell to the AKP.
Sarigul also called for a recount in Istanbul, while the CHP's Ankara mayoral candidate, Mansur Yavas, said his party would go to the constitutional court if necessary.
CLASHES IN THE SOUTHEAST
Despite a turbulent political past, previous elections in Turkey have been largely seen as free and fair.
The vote in southeastern Turkey, where a ceasefire has been holding since last year as part of an effort to end a three-decade insurgency by Kurdish militants, was marred by isolated violence. The pro-Kurdish BDP extended its control of provinces in the region, according to unofficial results.
Eight people were killed in two separate shoot-outs in villages in the southeastern provinces of Hatay and Sanliurfa near the Syrian border on the day of the vote itself during clashes between supporters of rival candidates for local office.
Riot police fired tear gas and water cannon to disperse protesters outside local government offices in the southeastern district of Ceylanpinar near the Syrian border on Tuesday after results there showed an AKP victory, security sources said.
The clashes were sparked by rumours of fraud, including the burning of ballot slips, according to Hurriyet. The pro-Kurdish BDP party was challenging the result.
The newspaper meanwhile said ballots stamped with votes for the CHP and the nationalist MHP party had been found in rubbish bags at six schools in a district won by the AKP in the otherwise MHP-dominated southern province of Osmaniye. (Additional reporting by Daren Butler in Istanbul, Umit Bektas in Ankara; Seyhmus Cakan in Diyarbakir; Writing by Nick Tattersall; Editing by Ralph Boulton and Tom Heneghan)