Catherine Stihler told a pro-EU rally in Edinburgh that a second vote on the final withdrawal agreement was necessary to “hold truth to power”.
She was speaking after Labour MP Owen Smith was dismissed from Jeremy Corbyn’s shadow cabinet for backing a second referendum.
Hundreds of pro-remain activists attended the rally organised by the Young European Movement Edinburgh and the European Movement in Scotland after marching through the city centre to demand a final vote on the Brexit deal.
Ms Stihler was joined by cross-party speakers inducing SNP MP Joanna Cherry, former Tory MEP Struan Stevenson and Green co-convener Patrick Harvie.
Addressing the crowd, she said: “Yes, there was a referendum - a non binding referendum, an indicative referendum which narrowly supported a Leave vote which had forces at play which are only now being exposed and uncovered.
“But when you know something will damage people, it’s your responsibility to hold truth to power and, as John Major said, hold truth to people.
“I want to see people have the choice of whether to accept the withdrawal deal or to keep what we have at the moment: the benefits we have of being a full member of the EU.
“A second referendum on the deal is the only fair course of action to ensure this government is held to account.
“As things currently stand, the Government looks as though they will park the difficult decisions to the transition period, a period when we will no longer be a member of the EU.
“It will not be clear what a future EU /UK relationship will look like until we are a third country with no influence no say over the rules which will affect our people.”
Ms Cherry said she “completely understood” the arguments for a second referendum.
She added: “In the last referendum Scotland voted overwhelmingly to Remain and that was ignored, so if there is going to be another cross-UK referendum on the EU there has to be some respect shown for the constitutional settlement and some respect built into it for the will of the Scottish people.”
Mr Harvie said: “I’m not willing as some Remain politicians are to say well we had the vote, we have to respect it, we’re going to give up and pretend that we believe the opposite of the truth.
“I’m not wiling to go along with those who say that we can have a deal that will give exactly the same benefits of the single market but unwilling to talk about one of the critical benefits, freedom of movement.
“I’m not willing to do that because I know there is hope. I intend to die a member of the European Union and not by moving, by staying here.
“We are not willing to give up on our fight to stay in Europe. If we have to fight to get back in we will, but we’re not resigned to that fate yet and we’ll oppose Brexit with every breath that we have.”