71% in Guardian/ICM poll say first minister won debate in which he accused no campaign leader of siding with Tories

Alex Salmond staged a dramatic fightback in the Scottish independence campaign after mounting repeated attacks on the credibility of no campaign leader Alistair Darling in a bloody confrontation in advance of next month's referendum.

Salmond scored a decisive victory in a snap poll for the Guardian by more than two to one after he goaded Darling, leader of the pro-UK Better Together campaign, accusing him of collaborating with a Tory government in London in a combative and ill-tempered referendum debate broadcast live across the UK from Glasgow.

"Why are you standing here defending Conservative policies on a joint platform with the Conservatives?" Salmond asked – provoking an immediate retort from Darling that he supported neither the Tories nor the Scottish National party.

Salmond repeatedly won applause from vocal yes voters after stating that the referendum in three weeks gave the country "the opportunity to take power into Scotland's hands". In a rallying cry to Scotland's 4.2 million voters, Salmond added: "It's time, our time. This is our moment. Let's use it now."

But the debate frequently descended into a shouting match with the two speaking over one another and the chair losing control of the discussion as they traded blows over the currency, oil and the NHS.

A snap poll of 505 voters in Scotland, conducted for the Guardian by ICM, gave Salmond a resounding victory. Some 71% of those who have decided backed the Salmond compared with 29% who backed Darling, giving the yes campaign a crucial lift just as about 750,000 postal votes for the referendum are due to be sent out on Tuesday and Wednesday.

Darling countered by accusing Salmond of spreading lies and half-truths about NHS cuts in England which were "beneath contempt". The former Labour chancellor accused the first minister of "using scare stories in order to make a point which has nothing to do with the referendum".

Salmond is "playing games" and cannot be trusted, Darling said. "He's asking us to take his word for it, on everything. No plan B for everything; trust what he says," Darling said.

The poll result marks a profound change of fortunes for the first minister and is likely to energise the pro-independence campaign, which is trailing the no campaign by 14 points in the polls. A Guardian/ICM snap poll immediately after the first live debate on 5 August gave Darling a victory of 56% to 44%.

But the latest ICM poll also suggested that Salmond's victory in the BBC debate, broadcast from the Kelvingrove art gallery and museum in Glasgow, made little difference to the overall levels of support for independence, with support for both campaigns rising by one percentage point.

With the number of undecided voters – the most important group of voters for the campaigns – falling by just two points, the number of yes voters only slightly increased from 44% to 45%, with no voters also rising one point from 46% to 47%.

Under pressure to win the BBC debate, an assured and confident Salmond issued a rallying cry as he made his closing statement. "We only have to believe in ourselves," he said. "This is our time; this is our moment. Let's seize it with both hands."

With Scotland's currency options again a significant issue, Salmond's officials were adamant that Darling's repeated attempts to skewer Salmond on his refusal to spell out an alternative currency plan had failed on this occasion. "Darling has got one trick and it didn't work tonight," Salmond said.

But Darling's advisers signalled that they would now attack Salmond for claiming he did not need one plan B when he had "three plan Bs". One Better Together official said: "The line 'I have three plan Bs' is a total disaster for yes. And that was precisely the tone which puts undecided women voters off."

In a far more assured performance since his faltering live TV debate against Darling on STV earlier this month, Salmond insisted Scotland was a prosperous country. He claimed a yes vote in the referendum would allow the Scottish parliament to resist policies such as the bedroom tax or a new generation of nuclear weapons.

Dismissing the "voices of doom" from anti-independence campaigners, Salmond added: "Absolutely no one will run the affairs of Scotland better than the people of Scotland, no one cares more than the people of Scotland."

Signalling a fresh attack on Salmond's perceived vulnerability on Scotland's currency options and future North Sea oil revenues, with increasingly bad-tempered exchanges between the two men, Darling insisted that the first minister needed to be clearer on those issues.

"Are we going to place our bets on Alex Salmond alone being right?" the Better Together leader said. "We might hear some good lines from him tonight, but remember a good line isn't always a good answer. Good answers are what we need."

Insisting that voting to stay in the UK would see Scotland get new powers over welfare and tax rates, while retaining the security and stability of membership of the UK, Darling said: "My first priority is to build a fairer and better society. His priority is to create a separate state, not matter what the risks or what the costs."

As Darling denied Salmond's claims that Scotland's separately-run NHS was under threat from privatisation policies in England, one audience member accused the former Chancellor of starting off the privatisation process and said that next time he ate at a private dinner for healthcare companies, "I hope you can see Aneurin Bevan [founder of the NHS] sitting on your shoulder."

The first selection of audience questions had immediately returned to one of the key subjects of the first debate: currency union.

Salmond insisted that a yes vote would give him a clear mandate from Scotland's votetrs to share the pound "in a sensible currency union with the rest of the UK" but – despite one questions that specifically asked him to lay out his Plan B – continued to avoid going into detail on alternative options.

Darling retorted that "yes there is the sovereign will of the Scottish people but there is also the sovereign will of the rest of UK."

A currency union would be bad for Scotland because it would have to have its budget approved by the rest of the UK, but then refused to imagine his own Plan B in the event of independence, describing them as "fallbacks that are second best and bad for Scotland."

Asked about oil, Alex Salmond insisted "the reality is North sea oil and gas will be with us way beyond 2050.Every other country in Europe would give its eye teeth to have North Sea oil and gas. It cannot be regarded as anything other than a substantial asset for Scotland." Darling accused him of "gambling our children's future".

The polling expert Professor John Curtice said before the BBC debate, broadcast from the Kelvingrove art gallery and museum in Glasgow, that Salmond needed the BBC event to be a "game changer" with time before referendum day ebbing away.

The latest poll of polls has found there is still a persistent 14 point gap between the no and yes vote, with the no vote on average at 57% compared with 43% for yes, despite another small rise in yes support, with just over three weeks to go before referendum day on 18 September.

Curtice said that a draw between the two men would be a satisfactory result for Darling, because of Better Together's consistent overall lead. "A draw is good enough for Mr Darling and Better Together and that is all he has to do tonight because of where they are in the polls," he said.

The debate also takes place just before post votes are sent out to as many as 750,000 voters on Tuesday and Wednesday – a record number, whose vote could be influenced by either leaders' performance at the Kelvingrove. Campaign officials report the highest levels of applications for postal votes yet seen, with the deadline for registering falling on 2 September.

Campaign sources say this will be the last leaders' debate between Salmond and Darling, with Sky and Channel 4 News, the broadcasters still bidding for a live debate, now planning debates involving campaign deputies and other party leaders in Scotland instead.

STV, which hosted the first Salmond versus Darling contest on 5 August, has announced it will stage a further major debate on 2 September, expected to involve three pro-independence and three pro-UK figures, lead for yes by Scotland's deputy first minister Nicola Sturgeon and for no by Johann Lamont, Scottish Labour leader.

Salmond is widely expected to pursue a far more negative strategy in this debate, attacking the UK government's track record on spending cuts, welfare reform and privatisation of NHS services in England, raising the threat of even greater austerity if the Tories win the next UK general election.

That approach, used to reinforce the idea that Scotland must become independent to protect itself, is described by SNP strategists as the "two futures" argument which contrasts the possible outcomes for Scotland in the event of a yes or a no vote.