Alex Salmond today challenged opposition parties to come up with an alternative to independence that could be put to the public in a referendum.

The First Minister's challenge came when he launched the latest stage of the Scottish Government's National Conversation on the country's constitutional future.

The SNP hopes to stage a referendum in 2010 on whether Scotland should become independent.

Mr Salmond today suggested the referendum having two other options, more powers for the Scottish Parliament or no change in the present set-up.

And he said that voting in that referendum could be carried out on a system of one-two-three preferences, as used in last year's local government elections.

This differs markedly from what was set out in a document launched last August when the National Conversation began.

That envisaged a simple yes or no answer to the question of independence, although Mr Salmond said at the time he was "relaxed" about other options being included.

A spokesman for Mr Salmond today denied the First Minister was breaking new ground by talking of a preferential voting system.

"In all the many years the SNP have talked out a constitutional multi-option referendum it has always been on the basis of a preferential voting system," said the spokesman.

The First Minister's challenge to rival parties came when he addressed a gathering in Edinburgh of civic groups from Scottish society, including churches, unions and universities.

Engaging "civic Scotland" in the debate on Scotland's constitutional future forms the second phase of the National Conversation which began last August.

A separate commission, chaired by former chief medical officer Sir Kenneth Calman, is reviewing the powers of the Scottish Parliament ten years on from devolution.

That commission has the backing of Labour, the Lib Dems and the Tories - and independence has been excluded from its deliberations.

Mr Salmond argued today there was now consensus that the present constitutional framework did not give Scotland the opportunities it needed to succeed.

"The question is therefore not whether we should have more powers in Edinburgh but what powers and within what timescale?" he said.

The First Minister said he was "relaxed" about the commission set-up by the other parties.

But he said: "I have already said that as First Minister that I am happy to test support for enhanced devolution, along with support for independence for Scotland.

"And I say to those who oppose the restoration of Scottish independence that just as I respect absolutely their right to hold that view, so in return I feel able to require of them a clear alternative which can be put onto a ballot paper and held up to public scrutiny and be available for a decision by the Scottish people.

"I say that not as a Nationalist but as a democrat."

Mr Salmond said that unlike the commission, the National Conversation was intended to be inclusive of other views.

He pledged to look seriously at whatever suggestions were made as part of the National Conversation - with one exception.

And that was that some powers of the Scottish Parliament could be passed back to Westminster, as suggested by Gordon Brown, as part of a new devolution arrangement.

Mr Salmond declared: "There is no popular mandate whatsoever for such a diminution of Holyrood - the will of the Scottish people is for the repatriation of more powers and responsibilities here in Scotland."

mfl Today's event in Edinburgh took place before an audience of around 120 people from bodies ranging from churches to universities, trade unions and business groups.

Mr Salmond declared: "We are entitled to have different opinions about the best constitutional choice for our nation - but as First Minister I today ask simply that those who oppose an independent Scotland assume also the responsibility of constructive opposition.

"This debate is bigger than all of us and bigger than any party.

"It goes to the heart of who we are and who we want to be as a country - a debate so fundamental and so important that it demands that the people of Scotland make the final decision in a referendum."

He later fielded questions from the audience on topics ranging from nuclear weapons to the economy.

It was during these questions that he suggested the possibility of a preferential voting system for the referendum.

"People managed to get their heads around voting one-two-three in STV (the local government elections last year) so I think we want one-two-three in a three-option referendum," he said.

"I'm pretty confident people in Scotland can manage three choices on a ballot paper."

A business questioner told Mr Salmond the business community believes Scotland has the capability to become independent but questioned whether this was feasible or advisable.

"It's about facts and figures, not political rhetoric, that the business community is interested in," he said.

Mr Salmond pointed to Scotland's long-term growth rate of 1.8% compared to 2.5% for the UK and 3.5% in some other countries and declared: "In a nutshell that is the argument."

Mr Salmond argued nothing in Scotland's history gave cause to believe that the country was intrinsically an underperformer, and the answer must lie in the way Scotland's economy was governed as part of the UK.