The tide is turning and today is the day that we the people reclaim our democracy. On the streets of London, hundreds of thousands of us will be marching on parliament in support of parliament. We reject the prime minister’s assertion that she is the tribune of the people – uniquely able to divine our interests, divinely entitled to rule. We reject her deal, which leaves us poorer and more isolated. We reject her government, which has humiliated us on the international stage and enfeebled us at home. Instead we say to MPs that now is the time to assert themselves, to reject her threats and her jibes and to give back to the people the final say on Brexit.

Remember why we are where we are. When I resigned from the National Infrastructure Commission to campaign against Brexit and for a referendum – just short of 18 months ago – it felt as though Brexit was a done deal. Everywhere I went I met people who – however they had voted in 2016 – were dismayed by the reality of Brexit but convinced there was no alternative. It was soft Brexit versus hard Brexit – not “no Brexit”, which is in truth the only alternative.

“I wish it wasn’t happening but we can’t stop it” was the refrain. This fatalism is now behind us. In October we saw the biggest political march on the streets of London in a decade. Up and down the country the European Movement has grown from a smattering of small associations to more than 150 branches, with tens of thousands of members. And as I write, more than three million people have signed a petition calling on the government to revoke article 50 (the up-to-date figure is here).

Theresa May is singularly unsuited for high office and lacks political talent. Her late-night rant from No 10 on Wednesday may be remembered as the worst broadcast ever from Downing Street.

But, in truth, it is Brexit that has defeated Brexit, not May. Brexit has always been an impossible project, except at the price of massive self-harm. The hard, populist right sold the myth of “taking back control” because they knew that no one would buy the reality: workers’ rights slashed, welfare broken, Ireland betrayed and Britain turned into a giant tax haven. Theresa May has much to answer for, but it is not her fault that she couldn’t square the circle of the Brexiteers’ lies: nobody could.

I expect today to be the biggest political march in living memory. It will be defined not just by its size but by its diversity. Young and old, of all ethnicities, from every corner of our country, speaking up and speaking out. Refusing to let our prime minister rule as a self-appointed monarch. Rejecting the narrowness, insularity and the extremism of Brexit. Demanding a democratic solution.

In years to come, historians will write approvingly of the British people’s passionate, peaceful and successful fight against Brexit. They will say that as our institutions buckled under the strain, Britain found its voice. And you will be able to say to your children and grandchildren: “I was there. I stood up for my country, I faced down the extremists, I protected your future.” So come today. Join us. Let’s get our country back.

• Andrew Adonis is a Labour peer