* Opinion polls suggest voters set to overturn strict ban
* Turnout may top successful gay marriage referendum
* Church has taken a back seat in campaigning
* Exit polls set to give first indication of result (Updates after polls close, latest turnout estimate)
By Padraic Halpin and Graham Fahy
DUBLIN, May 25 (Reuters) - Prime Minister Leo Varadkar said he was quietly confident that a high turnout in Ireland's abortion referendum on Friday would favour those seeking change in what two decades ago was one of Europe's most socially conservative countries.
Voters turned out in droves in blistering sunshine and turnout could be one of the highest ever for a referendum, potentially topping the 61 percent who backed gay marriage by a landslide in 2015, national broadcaster RTE reported.
Participation was high in Dublin but also in large towns and rural areas across the country as polls closed at 2100 GMT. Queues formed outside polling stations throughout the day and returning officers noted a large presence of younger voters.
Prior opinion polls suggested voters in the once deeply Roman Catholic nation were set to overturn one of the world's strictest bans on terminations. Analysts said a high turnout was likely to favour a "Yes" vote.
"Not taking anything for granted but quietly confident. A high turnout, I think, would be to the advantage of the 'Yes' campaign," Varadkar, who is in favour of change and has called the referendum a "once-in-a-generation" chance, told reporters earlier in the day.
The Irish Times newspaper said it will publish an exit poll shortly after polls close and RTE will publish its at 2230 GMT.
Vote-counting begins at 0800 GMT on Saturday, with the first indication of results expected mid-morning.
A high turnout would suggest both voter bases are fired up, meaning those who were coming off the fence would be decisive, said John McGuirk, spokesman for the anti-abortion Save the 8th umbrella group.
Voters were asked if they wish to scrap a 1983 amendment to the constitution that gives an unborn child and its mother equal rights to life. The consequent prohibition on abortion was partly lifted in 2013 for cases where the mother's life is in danger.
Ireland legalised divorce by a razor-thin majority only in 1995, but became the first country to adopt gay marriage by popular vote in a 2015 referendum.
But no social issue has divided its 4.8 million people as sharply as abortion, which was pushed up the political agenda by the death in 2012 of a 31-year-old Indian immigrant from a septic miscarriage after she was refused a termination.
"I think this issue is important because it's been 35 years since any person has had a choice to vote," said Sophie O'Gara, 28, who was voting "Yes" near Dublin's bustling 'Silicon Docks', home to some of the world's biggest technology firms.
"So many women have travelled across to England to take care of their family and healthcare needs and I think it's a disgrace and it needs to change," she said, referring to women who travel to Britain for abortions.
The fiercely contested vote has divided political parties, seen the once-mighty church take a back seat, and become a test case for how global internet giants deal with social media advertising in political campaigns.
Unlike in 1983, when religion was front and centre and abortion was a taboo subject for most, the campaign was defined by women on both sides publicly describing their personal experiences of terminations.
"Yes" campaigners have argued that with over 3,000 women travelling to Britain each year for terminations -- a right enshrined in a 1992 referendum -- and others ordering pills illegally online, abortion is already a reality in Ireland.
Although not on the ballot paper, the "No" camp has seized on government plans to allow abortions with no restriction up to 12 weeks into a pregnancy if the referendum is carried, calling it a human rights issue and a step too far for most voters.
"I think it's important that we protect the unborn babies, people don't care anymore about the dignity of human life. I've a family myself and I think it's really important," said John Devlin, a marketing worker in his 50s voting "No" near Dublin's city centre.
The Irish government's push to liberalise the laws is in contrast to the United States, where abortion has long been legal, but President Donald Trump backs stripping federal funding from women's health care clinics that offer abortions.
HOME TO VOTE
Videos shared on social media showed scores of voters arriving home at Irish airports from abroad. Ireland does not allow expatriates to vote via post or in embassies but those away for less than 18 months remain on the electoral roll.
As with the gay marriage referendum, those using the #hometovote hashtag on Twitter appeared overwhelmingly to back change. Many posted photos of themselves wearing sweatshirts bearing the "Repeal" slogan.
"Women and girls should not be made into healthcare refugees when they are in a time of crisis," said Niamh Kelly, 27, who paid 800 euros and travelled 20 hours to return home from Hanoi where she works as an English teacher.
"This is a once in a lifetime generation chance to lift the culture of shame that surrounds this issue so it was really important to me to be part of that."
Graphic on abortion policies worldwide: https://tmsnrt.rs/2Lu7DM7
(Additional reporting by Emily Roe; Editing by William Maclean and Angus MacSwan)
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