Leader Colin Fox, a former MSP, said that only with a Yes vote next year can Scotland's working class become better off, socially and politically.
"For the British state, Scottish independence represents a huge threat, a profound loss of economic and political power and influence at home and abroad, and consequently it is dead-set against it and will do its utmost to stop that from happening," he said inside the Scottish Parliament.
"For the independence movement, the stakes are equally high. The referendum offers a once-in-a-lifetime opportunity to secure self-determination for Scotland, to establish a left-of-centre, social-democratic state and free five million Scots from the yoke of British imperialism.
"I will put it to you that ours is the case for independence-max."
Mr Fox said his party can advance its cause despite having no MSPs at Holyrood.
As members of the Yes Scotland pro-independence campaign group, socialists can spread their message, he said.
Describing himself as a socialist, not a nationalist, he said Scotland should have control of its destiny.
Support for independence is part of the class struggle, he said.
"Scotland is a nation, not a province or a region of anywhere else, let alone England," he said.
"We have our own country, our distinct national identity, our own Church, our own education system, our own legal system, banknotes, sports team and culture.
"The United Kingdom on the other hand is not a country at all. It is a political union made up of at least four countries.
"I'm not a nationalist, I'm a socialist."
He "respects" the SNP which, he conceded, may be the main beneficiary of independence.
But the SNP is signed up to a capitalist vision of independence, he said.
"By voting Yes next September, we won't deliver any of these gains and reforms automatically but it will, in my view, undoubtedly bring the prospect of them far, far closer."