More than 140,000 people have signed up as supporters of Yes Scotland, the pro-independence campaign group said.
The cross-party organisation, set up to encourage people to vote for independence in the 2014 referendum, said some 143,000 had now backed it - more than the total number of Scots who are members of political parties.
Yes Scotland chief executive Blair Jenkins hailed the level of support as a "tremendous achievement this early in the campaign".
He said: "The figures show a real sense of momentum all around the country and we have lots of people who have never been involved in politics coming to us and putting their names down. There is still a long way to go until we cross the finishing line in the autumn of 2014, but reaching this number of declared supporters this early gives us great confidence for a positive result in the referendum."
Mr Jenkins added: "We've said from the outset that this will be the biggest campaign - a people's campaign - ever seen in Scotland and the numbers of voters getting involved supports that prediction."
Former Labour MP Dennis Canavan is playing a key role in the pro-independence campaign as the chairman of the Yes Scotland advisory board. He said: "This achievement this soon gives us fantastic optimism for the success of our campaign - and for the future of our nation."
Yes Scotland supporters sign a declaration which states it is "fundamentally better for us all, if decisions about Scotland's future are taken by the people who care most about Scotland, that is, by the people of Scotland".
Signatories to this have been gathered through the campaign group's website, canvassing, at special events throughout the country and through the Yes Scotland local groups, which are being set up in all 32 local authority areas.
A spokesman for the Better Together group, which is campaigning to keep Scotland in the UK, insisted the majority of Scots did not support independence. When the Yes Scotland campaign was launched in May, First Minister Alex Salmond said the aim was to have one million Scots sign their declaration.
The spokesman for Better Together said: "Their claims of momentum are a nonsense. At this rate, they will have managed to reach their target of convincing a quarter of the electorate exactly a year after the referendum. If they want to sign up enough Scots to win a majority, they will still be chasing signatures in 2018. The reality is that the overwhelming majority of Scots are not buying what the separatists are selling."