Alistair Darling accused Alex Salmond of arguing for independence using "guesswork, fingers crossed and blind faith" in a combative television debate that an instant poll said was won by the former chancellor.

An ICM snap poll of more than 500 Scottish voters for the Guardian said Darling, leader of the Better Together campaign, won the live debate with Scotland's first minister by 56% to 44% – defying expectations that Salmond would be the victor.

The verdict, where viewers were asked to set aside their own views on independence, came after the two men clashed angrily over the fate of the country's currency, the economy and the imposition of the bedroom tax and nuclear weapons.

Alistair Darling debating for the no campaign. Photograph: Reuters

Salmond had insisted: "No one will do a better job of running Scotland than the people who live and work in this country," later adding: "It's a vote for ambition over fear; this is our moment, let's take it."

Darling said: "I don't want to see our children's future gambled away."

The long-awaited TV confrontation between the leaders of the yes and no campaigns was staged over 90 minutes on STV, and was only shown online outside Scotland, although some viewers reported difficulties when trying to watch the stream via the internet.

During bitter exchanges between the two men, in front of 350 yes, no and undecided voters at the Royal Conservatoire school of music and drama in Glasgow, the first minister found himself jeered by some audience members as he swerved questions over his plans for a Scottish currency after independence.

Darling was in turn booed as he dodged questions on Labour's plans to give Scotland full control over income tax and whether he agreed that an independent Scotland could be a success.

Salmond began his own interrogation of Darling by asking: "Why does the no campaign call itself Project Fear?" To cheers from yes supporters, he goaded his rival over the collapse of the UK economy while Labour was in government, saying: "You were in charge of financial regulation when the banks went bust."

Darling retaliated by repeatedly pressing an increasingly harried Salmond on his plans for Scotland's currency, given all the UK parties had emphatically rejected a sterling pact after a yes vote: "Any eight-year-old can tell you the flag of a country, the capital of a country and its currency.

The debate was hosted by Bernard Ponsonby at the Royal Conservatoire of Scotland on 5 August 2014 in Glasgow, Scotland. Photograph: Jeff J Mitchell/Getty Images

"I presume the flag is the saltire, I assume our capital will still be Edinburgh, but you can't tell us what currency we will have. What is an eight-year-old going to make of that?"

Salmond had gone into the debate buoyed by new poll findings from Ipsos Mori for STV that found the yes vote had risen by four points to 40%, with more undecideds switching to back independence, while the no vote remained static at 54%.

The ICM poll for the Guardian found a narrow majority in favour of staying in the UK, at 47% against independence versus 42% voting yes, and despite finding that Darling had won the debate, most voters thought Salmond had the most attractive personality, at 47% to 39%. But on the critical issue of who had the better arguments, 51% said Darling was the strongest, with 40% backing Salmond.

Alex Salmond debating for the yes campaign. Photograph: Peter Devlin/AFP/Getty Images

His advisers now fear the first minister, widely expected to dominate the exchanges, failed to seize the opportunity with only 43 days to go before the referendum on 18 September.

With the yes campaign only slowly closing the gap in the polls, it conceded Salmond did not win the debate, but insisted its strategy of talking directly to swing voters at home had succeeded: "We think he used the opportunity to get our message across. We think he was successful."

The Darling team claimed victory for Better Together: "The yes side claimed it would Darling's Bannockburn but it turned out to be Salmond's Waterloo. Alistair won because of his forensic questions on currency."

Salmond had won the toss to speak first, and launched a vigorous attack on the growing number of food banks in Scotland and the £100bn he said had been committed by the UK government to keep nuclear missiles on the Clyde.

The first minister said there were now 35 food banks in the Glasgow area, despite the country's great wealth, while the country was expected to pay £8bn for a nuclear weapons system it had rejected.

Repeating now-common themes for the yes campaign, Salmond said for half his lifetime his country had been governed by parties Scotland had not elected, referring to policies such as the poll tax and the bedroom tax, "imposed on us by Westminster that Scottish MPs voted against, but did not have the power to prevent".

The first minister said: "My case this evening is this: no one, no one will do a better job of running Scotland than the people who live and work in this country." Darling retorted in his opening statement that every Scot and all no voters wanted Scotland to prosper. He said it was not his patriotism that was being tested but independence would fix the fate of future generations.

"If we decide to leave, there's no going back," he said. "There's no second chance. For us, the choice is very, very clear. I want to use the strength of the UK to make Scotland stronger. [A] vote against independence isn't a vote for no change."

Salmond speaks to the media after the debate. Photograph: Jeff J Mitchell/Getty

He said all three UK party leaders had already pledged to boost Scotland's already substantial devolved powers over income tax and social security. Darling said: "Let's say it with confidence, let's say it with pride, let's say it with optimism, no thanks to the risks of independence."

Yet Darling too was jeered by pro-independence campaigners as he dodged a question from the debate host, Bernard Ponsonby, about whether Holyrood would get total control of income tax in Scotland: Labour has rejected that proposal, which is backed by the Tories and Lib Dems.

The former chancellor said Holyrood would get substantial powers over the tax, but was left unable to specify them.