(Adds turnout data, paragraph 10)
* Modi posts "selfie" of party symbol on eighth voting day
* Rules ban political use of media for 48 hours after vote
* Opposition candidate Modi favoured to become PM
* Ruling party focuses on denying majority to Modi's party
By Aditi Shah
AHMEDABAD, India, April 30 (Reuters) - Police opened an investigation against Hindu nationalist Narendra Modi, tipped to be India's next prime minister, after he flashed his party's symbol and made a speech in a violation of election rules after he cast his ballot.
About 139 million people were registered to vote in the eighth round of a marathon contest pitting Modi against the ruling Congress party, led by the Nehru-Gandhi dynasty. Results are due on May 16.
Voting in his home state of Gujarat, the opposition leader, whose pro-business policies have delighted investors, brandished a white cutout of a lotus flower and made a scathing speech against Congress heavyweights - taunting them for shying away from the fight.
"The prime minister himself is not fighting the election. The finance minister is not fighting the election. All its top leaders have run away," Modi said to cheers from a crowd at the polling station in the state's largest city, Ahmedabad.
He snapped a "selfie" of the lotus and his finger painted with ink after voting, and posted the photograph on Twitter.
Election rules say politicians must not make public rallies or use media to "display to the public any election matter" within 48 hours of an election.
Gujarat police chief PC Thakur said a preliminary case was launched against Modi at the request of the election commission. "The Ahmedabad crime branch has begun investigations," he said.
The maximum punishment for violating the rule is two years imprisonment, although Modi is unlikely to be charged. Politicians in India routinely face criminal cases that rarely reach the courts.
Standing in both the Gujarat town of Vadodara and the holy city of Varanasi, Modi has shaken up Indian politics with a campaign that has combined a social media blitz with up to five rallies a day. The 63-year-old has even appeared as a hologram campaigning in remote hamlets.
Turnout in Gujarat was 62 percent on Wednesday, according to the election commission, a sharp rise on the state's tally of 48 percent in the last general election in 2009. India has seen higher voter turnout across most states so far in the staggered election.
Opinion polls give a coalition led by Modi's Bharatiya Janata Party (BJP) a strong lead and predict the worst ever result for Congress, which led India to independence from Britain in 1947 and has dominated politics ever since.
But most surveys predict the BJP will fall short of the 272 seats needed for a parliamentary majority, meaning it will need to find allies. The size of the shortfall will determine whether a Modi government can pass free market reforms aimed at reviving the economy, or be constrained by protectionist allies.
The BJP "will almost certainly beat the Congress," said Nida Ali of Oxford Economics.
"Now they are trying to maximize the number of seats they can get, so they are not hindered by other parties. If they can get a majority, that would help in decision-making."
Indian shares have risen 6.5 percent in 2014 through Tuesday, outperforming the 0.5 percent drop in the MSCI emerging equities index, on expectations the industry-friendly BJP would score an emphatic win. But shares have cooled of late as traders turned cautious ahead of election results.
The results of Indian elections are notoriously hard to predict, with bloc voting by caste and religion. Dramatic last-minute swings can confound experts: opinion polls got the result wrong in 2004.
In a reminder of the difficulties in converting Modi's popularity into seats, Arun Jaitley, a possible future finance minister, risks losing a contest in the state of Punjab over anger with the state government headed by a BJP ally.
The BJP's president, Rajnath Singh, also faces a tough fight in Lucknow, capital of the big state of Uttar Pradesh, where voters lined up at schools on Wednesday despite the blazing summer sun.
The election remains Modi's to lose, however, and in recent days several senior Congress leaders have appeared to concede that prospects are gloomy. Finance Minister P. Chidambaram said on Monday that "crucial mistakes" were made as public anger over corruption rose in 2010 and 2011.
The Congress party has governed for two terms and oversaw some of India's fastest ever growth, but lost popularity as the economy slowed and rampant graft was uncovered.
Chidambaram himself chose not to contest this election, a decision seen by many as a sign of weakness, while Prime Minister Manmohan Singh is retiring.
A top adviser to Congress president Sonia Gandhi told the Times of India on Monday that the party would consider backing a non-BJP coalition led by a different party to stop Modi.
The party has since distanced itself from the comments.
"The Congress party and its allies will form the next government at the centre," said Shakeel Ahmed, a party general secretary.
Congress waged a lacklustre campaign, led by Rahul Gandhi, the great-grandson of India's first prime minister Jawaharlal Nehru. Gandhi's mother, Sonia, has also been a prominent campaigner, as has his sister. Some party leaders have even hinted that a spell in opposition would be welcomed.
Modi wants to break the hold of the dynasty on Indian politics once and for all. He appealed to voters to put a strong government in place.
"The voting that has happened has achieved two things. One, the mother-son government is gone. Second, a new government with a strong foundation will be in place," he said.
(Additional reporting by Sruthi Gottipati, Manoj Kumar, Rajesh Kumar Singh, Shyamantha Asokan and Malini Menon in NEW DELHI and Sharat Pradhan in LUCKNOW; Writing by Frank Jack Daniel; Editing by Douglas Busvine and Robert Birsel)