* Thousands protest local election results
* Erdogan's AK Party dominated electoral map
* Opposition challenges counts in key cities (Adds Istanbul opposition candidate, details)
By Jonny Hogg
ANKARA, April 1 (Reuters) - Riot police fired water cannon in the Turkish capital on Tuesday to disperse thousands of people protesting outside the Supreme Electoral Council (YSK) against local election results which saw the ruling party dominate the electoral map.
Prime Minister Tayyip Erdogan's Islamist-rooted AK Party kept control of the two biggest cities, Istanbul and Ankara, and increased its share of the national vote in Sunday's elections despite a corruption scandal dogging his government, but the opposition has said it will contest some of the results.
The crowd, calling for a recount of the Ankara result, which was particularly close, chanted "Thief Tayyip!" and "YSK, the people are with you!" before the riot police moved in.
Opposition supporters, many of them students who answered calls on social media, meanwhile 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."
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 between 26-28 percent for the CHP.
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, which, despite raising its share of the national vote, failed to shrug off its image as a bastion of the secularist elite, aloof from the realities of life for the weight of the population in the socially conservative Muslim 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 which fell to the AKP. Sarigul also called for a recount in Istanbul.
The YSK has said it will only announce final, official results once all claims of irregularities have been resolved. That could take weeks, with the CHP filing 8,000 complaints in Ankara alone according to the Hurriyet daily, four times the challenges nationwide at the last local elections in 2009. (Additional reporting by Daren Butler in Istanbul; Writing by Nick Tattersall, Editing by Angus MacSwan)
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