Thousands of independence supporters have begun marching through the streets of Glasgow.

The march is the first of eight planned for 2020 by the grassroots organisation All Under One Banner (AUOB) in what is likely to be a crucial year for the Scottish independence movement.

It comes the day after the former Labour cabinet minister and MP Ben Bradshaw declared he was “100% certain” that Scotland would choose to leave the UK in the event of a second referendum on independence, which the first minister and SNP leader, Nicola Sturgeon, is pushing for later this year.

Bradshaw told the German newspaper Der Tagesspiegel: “The political class in England – and this includes my own party – must very quickly recognise the Scots’ right to self-determination.” His remarks were tweeted approvingly by Sturgeon.

AUOB, which says it is not aligned with any political party, will organise events across the country, in Peebles, Elgin, Kirkcaldy, Stirling, Edinburgh and again in Glasgow, with the next march planned for Arbroath in April to mark the 700th anniversary of the signing of Scotland’s declaration of independence.

The march departed from Kelvingrove Park at 11.30am, and is heading to Glasgow Green, where it will end.

On Thursday, organisers were forced to cancel a rally planned for the Green after forecasts of high winds resulted in safety concerns.

The AUOB organiser Gary Kelly said that, before the weather warnings were in place, he had applied to Glasgow city council with an estimate that about 100,000 would attend the march. He said the programme of marches this year reflected a growing momentum for a second independence referendum.

“After the general election, where the SNP got 44% of the vote in Scotland, how many more mandates do they need? I’m really hopeful for an independence referendum this year, but I have a feeling that it may be dragged into 2021 because of the Holyrood election.”

During the general election campaign, the UK Labour leader, Jeremy Corbyn, said if his party won it would consider granting the powers to hold a second referendum if there was a pro-independence majority after next year’s Scottish parliament elections. Boris Johnson has consistently said he would refuse to do likewise regardless of electoral circumstances.

Scotland’s first minister, Nicola Sturgeon, wrote to the prime minister at the end of last year requesting the transfer of the necessary section 30 powers to hold a second referendum, and also called for the Scottish parliament to be given permanent powers to hold subsequent referendums on independence from the UK.

Describing the SNP’s success in the general election as an “unarguable mandate by any normal standard of democracy”, Sturgeon also published a 38-page document titled Scotland’s Right to Choose, to which Johnson has said he will give “careful consideration”.

On Thursday, the Guardian revealed that Scottish Labour was considering backing a second independence referendum in a reversal of policy by the party leader, Richard Leonard.

Leonard wants to hold a special conference in May to decide Scottish Labour’s position on a referendum, including the possibility of asking for a pro-federal option in a multi-option ballot.