UPDATE 3-In Scotland, pro-independence leader flunks TV debate
* Scotland to hold secession vote on Sept. 18
* Up to a million voters still undecided - polls
* Two campaign leaders clash in first TV debate
* Poll of viewers shows Salmond lost the debate
* Pro-UK leader warns over currency, economic future (Recasts, updates with reaction)
GLASGOW, Scotland, Aug 5 (Reuters) - The leader of Scotland's campaign for independence failed to turn a U.S.-style television debate into a victory for his cause on Tuesday six weeks before Scots vote on whether to break up the United Kingdom.
In an unexpected setback for those who support a breakaway, Alex Salmond, the leader of the pro-independence Scottish National Party (SNP), didn't land a knockout blow in a lively debate against Alistair Darling, the leader of the campaign to keep Scotland in the UK.
With the pro-independence camp trailing in opinion polls ahead of a Sept. 18 referendum that will decide whether Scotland breaks its 307-year union with England, most commentators had predicted Salmond, a powerful speaker, would notch up a rhetorical victory to breathe new life into his campaign.
By contrast, Darling, a former British finance minister with the manner of a school master, had been expected to flop.
But many observers said the 59-year-old Salmond had not performed as well as expected and critics said he had failed to craft a convincing economic vision for an independent Scotland.
An ICM poll of viewers for the London-based Guardian newspaper showed Salmond had lost the debate, while Ladbrokes bookmakers lengthened the odds on Salmond securing independence to 4/1 from 7/2. Odds on a no vote remained unchanged.
"My case this evening is simple: No one, absolutely no one, would do a better job of running Scotland than the people who live and work in Scotland," Salmond told an audience in Glasgow in front of a screen bearing Scotland's white and blue flag.
"On the 18th of September we have the opportunity of a lifetime - we should seize it with both hands," said Salmond, who was peppered with questions from pro-UK campaign chief Alistair Darling about plans for Scotland's currency.
The ICM poll showed 47 percent of those who watched the debate thought Darling had won while 37 percent thought Salmond had won. Fifteen percent said they didn't know. A total of 512 people were polled.
"I don't think it was Alex Salmond's best night," said Iain Macwhirter, a political commentator for the Herald newspaper in Scotland. "Sometimes Alex Salmond's attempt to be statesmanlike looked liked complacency and I am not sure that will have gone down terribly well with the voters."
Surveys consistently show opponents of independence holding on to a substantial lead over those who want to end the union with England, though as many as a quarter of Scotland's 4 million voters have yet to decide.
A poll from Ipsos MORI released as the TV debate commenced showed support for independence had risen to 40 percent, up 4 percentage points since a similar poll in June and the highest support that the pollster has yet recorded for the "Yes" campaign.
"There was no clear winner, but Darling probably did better than the expectation was at the beginning," Mark Diffley, Ipsos MORI's research director in Scotland, told Reuters. "A lot of people thought it would be very one-sided and that Salmond would be a clear winner, and I don't think that's the case at all."
Salmond argued that an independent Scotland could build a fairer and richer society and kept calm when repeatedly questioned about his plans for the economy.
He told the audience the British government spent far too much on nuclear weapons and said it had failed the people of Scotland. Salmond has promised to rid Scotland of nuclear arms if Scotland becomes independent.
"If we decide to leave, there is no going back, there is no second chance. For me the choice is very very clear: I want to use the strength of the United Kingdom to make Scotland stronger," countered Darling, the 60-year-old head of the "Better Together" anti-independence campaign.
Darling, a silver-haired member of the last British Labour government, focused on economic arguments, particularly what plans Salmond had for its post-independence currency and its future revenues.
Darling, at times raising his voice, pushed Salmond hard on how an independent Scotland could keep the pound, given that the British government had excluded a currency union.
"What is plan B?" Darling repeatedly asked. "That's using sterling like Panama or Ecuador uses the dollar."
"I am in favour of keeping the pound sterling," Salmond, dressed in a dark grey suit, said, after he was booed by at least one member of the audience.
Darling was spoken over by at least one member of the audience and the moderator of the debate at one point asked the audience to respect the debate. Both men interrupted each other.
Sometimes reading from notes and quoting news reports, Salmond branded the "No" campaign as 'Project Fear', one of his favourite criticisms, and complained about its tactics.
He also accused Darling, a left-wing politician, of being allied with ministers belonging to right-leaning Prime Minister David Cameron's government, which he said wanted to lead Britain out of the European Union.
"I want Scotland to stay inside the European Union," said Salmond. Cameron says he wants to keep Britain inside a reformed EU.
When asked about the perception that Salmond had lost the debate, a spokesman for his campaign said: "Darling was there to knock, knock, knock. I don't think he said a positive word in the whole two hours."
Voter reaction in the audience was mixed.
"I really felt though that Alistair Darling didn't put up a very good case. I was an undecided voter, and I've decided now that I'm going to vote for Scotland," Tom Hunter from Glasgow told Reuters, saying he'd vote for independence.
Salmond's supporters argue that Scotland, which has its own parliament and judicial system but lacks substantial tax-raising powers, would be freer, better governed and richer on its own.
The "No" campaign argues Scotland would be unable to keep the British pound, that tens of thousands of jobs in the defence and financial sectors would be at risk, and that an independent Scotland might find it hard to rejoin the European Union.
Only Scots living in Scotland can vote in the referendum and only viewers north of the border with England were able to watch the debate on terrestrial TV.
An affiliate of British broadcaster ITV showed the debate in Scotland. In the rest of Britain, it broadcast a gardening show at the same time. Some Scots living in England tried but failed to watch the debate on the broadcaster's Internet site. (Writing by Guy Faulconbridge and Alistair Smout; Additional reporting by William James; Editing by Andrew Osborn)