By Sabina Zawadzki and Alexander Tange
COPENHAGEN (Reuters) - Denmark's election was too close to call as voting continued on Thursday, a first exit poll said, with Prime Minister Helle Thorning-Schmidt's center-left alliance running neck-and-neck with the main opposition.
Thorning-Schmidt's alliance has drawn level in the polls with the opposition center-right in recent weeks, after trailing, sometimes far behind, due to unpopular reforms and charges of broken promises since taking power in 2011.
The surge emboldened her to call the election, gambling that an economic revival would secure her a new term.
The opposition alliance, led by former prime minister Lars Lokke Rasmussen, is promising tax breaks and tougher controls on immigration. He will have to rely on support from a right-wing, anti-immigrant and Eurosceptic group.
"It's going to be really close," Denmark's first female prime minister said as she voted in Copenhagen accompanied by her husband Stephen Kinnock, a member of the British parliament. She urged voters to trust her economic and welfare policies.
A YouGov exit poll for the Metroxpress free daily, published while voting was still under way, gave Rasmussen's bloc a tiny advantage of 50.4 percent to 49.5 percent for Thorning-Schmidt. It said the result was too close to call.
Exit polls have often proved unreliable in previous Danish elections.
Earlier on Thursday, a weighted average of recent polls also showed Rasmussen's center-right ahead by a fraction. But one of the last polls indicated a slim lead for Thorning-Schmidt.
Rasmussen said he would push for reforms to the European Union against deeper integration, backing attempts by British Prime Minister David Cameron to change its relationship with the EU. "I currently see a window of opportunity" for EU reform, he said as he voted.
Danes in central Copenhagen reflected the split over the early election, called by Thorning-Schmidt before a September deadline because of signs of an economic revival. Growth of 1.7 percent is forecast for this year.
"We don't need lower taxes. The rich will survive," said Peter Gaub, 29, saying he was voting for Thorning-Schmidt.
Britta Andresen said she favored Rasmussen because of his tougher line on immigration. "You're welcome to come but you have to fit in and work," she said.
If the polls showing a knife-edge race are correct, the vote may become a repeat of a 1998 election that was so close just 180 votes from the Faroe Islands tipped the balance in favor of the center-left.
In a Nordic country known for consensus politics, differences between the two blocs have more to do with their leaders' styles -- both are accused of extravagance, with nicknames of "Gucci Helle" and "Luxury Lars" -- rather than their policies.
Rasmussen's Liberals want to freeze state spending and introduce tax breaks; Thorning-Schmidt's Social Democrats have pledged increased spending.
"Especially from a financial-market point of view, there's no difference -- both sides are in favor of our fixed exchange rate policy and conducting a responsible fiscal policy," Danske Bank Chief Economist Steen Bocian said.
Thorning-Schmidt's side trailed Rasmussen's by as much as 17 points over the past four years. But it has caught up thanks to a reviving economy, her dignified response to an Islamist shooting attack and her high standing among fellow EU leaders.
Uncertainty lingers about the role of the right-wing Danish People's Party which has surged in popularity in recent years and came out top in European Parliament elections last year. It is unclear if it will push for cabinet seats if Rasmussen wins.
Polls close at 1800 GMT followed immediately by two exit polls. Counting in Denmark is expected to finish at 2200 GMT.
(Writing by Sabina Zawadzki and Alister Doyle; Editing by Catherine Evans)