Billionaires and Bollywood stars took their turn with Mumbai's slum dwellers on Monday to vote in India's marathon election as Prime Minister Narendra Modi steps up his nationalist campaign to sway the 900 million electorate. India's film and finance capital took centre stage in the fourth of seven rounds of voting with 72 constituencies across the nation taking part. Final results are to be released on May 23. At least 40,000 police and security personnel were deployed across the city of 20 million people while workers were given a holiday so they could vote for Mumbai's six lawmakers. Long queues were waiting when the 10,000 polling stations opened at 7.00am (0230 GMT). Media scrums were set up at the stations where India's richest man, Mukesh Ambani, Bollywood superstars including Amitabh Bachchan and Aamir Khan, as well as Indian cricket legend Sachin Tendulkar were due to vote. Former MP Priya Dutt, a member of one of India's most famous acting families, is trying to make a comeback for the opposition Congress party in Mumbai after sitting in the Lok Sabha assembly - the country's lower house of parliament - from 2005 to 2009. Award-winning actress Urmila Matondkar is running for Congress in the north of the city. The concerns of the rich and famous were far from the ordinary voters in line however. "The most pressing issue for Mumbaikars is unemployment and rising costs of living," shop trader Jignesh Shah said as he queued in the suburb of Chembur. "As a middle class family it gets really difficult when real estate costs are so high and transportation is a mess," the 56-year-old added. About 60 per cent of Mumbai's population live in slums. At the other end of the wealth spectrum, Mr Ambani has raised eyebrows by backing the opposition Congress candidate in his Mumbai constituency, which Mr Modi's conservative Bharatiya Janata Party (BJP) has left to an allied right wing party. He has regularly spoken in support of Mr Modi's BJP government and the Reliance Industries tycoon's son, Anant Ambani, attended a recent Modi rally. Monday's voting in nine states will be particularly crucial for Mr Modi's chances of securing a second term after his landslide win in 2014. More than three dozen seats are at stake in Madhya Pradesh, Uttar Pradesh and Rajasthan states, all traditional BJP Hindu strongholds. Defeats suffered by the ruling party in three recent state elections has raised Congress hopes that it can eat into the government's majority. Mr Modi's party won 282 of the 543 seats contested last time but analysts say this election will be closer. Congress has hit out at Mr Modi's handling of the economy, the failure to produce promised jobs and massive debts in rural India which has seen thousands of farmers commit suicide in the past four years. The 68-year-old prime minister, still seen as the frontrunner, has increasingly relied on his tough security stance and his nationalist credentials. At a rally in Mumbai on Friday, Mr Modi said only his BJP government can guarantee safety for the city, still recovering major militant attacks 10 years ago. "Modi is confident those who strike with terror on this city will be hunted down. That is how a city and that is how a country is run," said the prime minister who has given himself the nickname of "the watchman". Heavy security has been deployed across the country for the election and police baton-charged rival BJP and followers of the regional Trinamool Congress party in West Bengal state, officials said. Fighting broke out in the Asansol constituency over security at polling booths, said Amit Jyoti Bhattacharya, a spokesman for the state election commission. Trinamool Congress activists surrounded the car of local BJP candidate Babul Supriya and smashed its windows. "Police used batons to control the situation," the spokesman added.