Former UK chancellor Alistair Darling has launched the bid to keep the Union, saying there will be no way back from Scottish independence.
He compared independence to buying "a one-way ticket to send our children to a deeply uncertain destination".
Mr Darling argued Scotland could have the "best of both worlds", with a strong parliament at Holyrood and a secure place in the United Kingdom.
The Scottish government is planning an independence referendum in autumn 2014.
The SNP-led campaign for independence - Yes Scotland - got under way last month.
Launching the cross-party Better Together campaign in Edinburgh - which involves Labour, the Lib Dems and the Conservatives - Mr Darling said that Scotland, through family, economic and cultural friendship, was stronger in the UK.
He said he believed that Scotland was facing a crucial vote.
Mr Darling explained: "When Scotland votes in the referendum, we will face a historic choice which will shape our country and our families' futures, not just for the lifespan of a parliament, but for generations to come.
"Chairing this campaign, is one of the most important things I have ever done in politics - the decision we make is the most important we will make in our lifetime."
The Labour politician added: "We make a positive case for staying together. A positive case that celebrates not just what makes us distinctive but also celebrates what we share.
"We put the positive case for staying together. We are positive about our links with the rest of the United Kingdom, through families and friendships, through trade and through shared political, economical and cultural institutions.
"We're positive about being a proud nation within a larger state and the far wider range of opportunities for our people that this creates.
"We're positive about all of the identities that we share - Scottish, British, European, citizens of the world - and don't see the need to abandon any of them."
After Mr Darling's address, a promotional video was played featuring 40 people who said they backed the pro-Union campaign.
A number of those advocates were then interviewed by former Scottish Tory leader Annabel Goldie and Labour MSP Kezia Dugdale.
Scotland's Lib Dem leader Willie Rennie, who gave the final address of the launch, told the gathering: "Your decision is not really just for you, it's for future generations too - it is a big responsibility to make a decision about what legacy you want to leave for future generations.
"But it is a decision that we will take together - throughout this campaign, you will hear why we are safer, stronger and better together - you will hear from all three parties here today but more importantly you'll hear from individuals like the people here today.
"From every corner and every community in Scotland we will take nothing for granted in the campaign to keep our family together, we will work for every single vote."
'Exposed as a fraud'
Responding to the Better Together campaign launch, SNP leader and First Minister Alex Salmond said Mr Darling's "threadbare" case against independence was "exposed by the weakness of his arguments, and his presentation suffered from three devastating failures".
He explained: "Firstly, he claimed that the Union was a 'celebration' of Scottish values, on the very day that the Prime Minister of that political union is proposing to eliminate housing benefit for young Scots.
"Secondly, the anti-independence campaign's claim to be making a positive case has been exposed as a fraud. Alistair Darling's presentation was littered with words such as 'borders', 'division' and 'upheaval' - expressing arguments better suited to the 18th century than to the 21st.
"Thirdly, Alistair Darling said not a single word about the anti-independence campaign's alternative vision of more powers for the Scottish Parliament.
"That is because - at heart - this is a Tory-led campaign, which is intent on conceding nothing to the people of Scotland. Alistair Darling is operating as the frontman for a Tory-led campaign."
The Better Together launch came as half-a-million leaflets were handed out at train stations, setting out the case for keeping Scotland in the UK, while a new website went live on Monday.