Nicola Sturgeon has claimed independence is "within touching distance" ahead of a speech to supporters at a major rally in Glasgow.
She will ask for powers to hold another a referendum on Scotland's future in the UK shortly after next month's general election.
However Jeremy Corbyn said a new Scottish independence poll was not "desirable or necessary".
The Conservatives and the Liberal Democrats also oppose a further vote.
Ms Sturgeon is one of a number of SNP politicians and independence campaigners speaking at the #indyref2020 rally in George Square.
It will be the first time she has spoken at an independence rally since 2014.
The event prompted a counter demonstration by dozens of unionist supporters who waved flags and blew whistles as supporters of Scottish independence gathered.
Speaking ahead of the event organised by The National newspaper, the SNP leader focused on the UK-wide election on 12 December.
"This election really is the most important one Scotland has faced in modern times," she said.
"So much is on the line - people are completely fed up with the mess at Westminster.
"But George Square will be packed as people from all backgrounds join together to demand a better future for Scotland."
Ms Sturgeon added that independence "really is within touching distance".
She had faced criticism by some activists for not attending events such as the All Under One Banner march in Edinburgh last month.
However, the first minister did tweet ahead of that march to say she was not able to attend, but would be there "in spirit".
On Friday the first minister confirmed that she would send a letter "before Christmas" to whoever is in 10 Downing Street, requesting the Scottish Parliament is granted powers to hold another independence referendum.
She has made clear that she wants to hold a poll on the issue next year.
Asked whether she believed Labour would grant the Section 30 order, Ms Sturgeon answered: "Yes".
"If people in Scotland demonstrate the desire - as I believe they will in this election - for an independence referendum, then I don't believe Westminster opposition to the principle or to the timetable to that will prove sustainable," she said.
In response, Jeremy Corbyn said only a Labour government would be able to boost Scotland's economy and see "the levels of poverty in Scotland, particularly in the big cities of Glasgow, Edinburgh and Dundee, being reduced".
He added: "Scottish independence would mean a massive gap between what Scotland raises in taxation and what the Scottish people need at the present time.
"I think the much better option is a Labour government for the whole of the UK."
The Tories criticised Nicola Sturgeon for prioritising indyref2 "above all else".
Scottish Conservative MSP for Glasgow, Annie Wells, said: "While Nicola Sturgeon is banging on about indyref2, I'm out talking to people about the state of their local schools, the drug deaths crisis and violent crime taking over our streets, and the problems at the Queen Elizabeth Hospital.
"Instead of tackling the day-to-day things that Glaswegians care about, Nicola Sturgeon is headlining a nationalist rally.
"So this election is about stopping Nicola Sturgeon from dividing our communities all over again, and only a vote for the Scottish Conservatives will do that.