Jeremy Corbyn has declared "today is only the beginning" in a message to mark Brexit Day.
Labour's leader warned the battle will go on when trade talks begin in March between the UK, EU, US and other countries around the world.
In the 1,317 days since Britain voted to leave the EU, Labour backed triggering Article 50 and a soft Brexit but then moved to backing a second referendum, with many senior figures backing Remain.
Mr Corbyn accepted "there will be different views and feelings" about Brexit today but said the question now is the UK's future direction - with just 11 months to get an EU trade deal before the transition period expires in December.
"Whatever side we took in the Brexit debate we now need to bring the country together to shape our common future," he said in a video on social media.
"Today is only the beginning. Trade negotiations with Europe aren’t scheduled to start until March.
"We will hold the government to account every step of the way to protect jobs and living standards, to guarantee the rights of EU nationals here in the UK, and of UK nationals in Europe and to defend protections for workers and our environment.
"And we will resist a toxic Trump deal that puts our NHS, food safety and jobs at risk.
"The choice of which path we take for a post-Brexit Britain now lies before us."
Boris Johnson will tonight pledge to “bring this country together” in his own video message to the nation.
But at the same time, Brexiteers will be popping local sparkling wine corks and tucking into Yorkshire puddings as they celebrate Britain's exit in 10 Downing Street.
The PM will claim leaving represents “a moment of national renewal and change” and that – after 10 years of Tory rule – he will finally begin “levelling up” spending across the UK.
He knows many in the North and Midlands who “lent” Tories their votes to deliver Brexit have high expectations on domestic policy he may not be able to deliver.
There are also concerns over whether he can make good on his promise of a trade deal.
The PM will say: “Our job as a Government – my job – is to bring this country together and take us forward. And the most important thing to say tonight is that this is not an end but a beginning.
“This is the moment when the dawn breaks and the curtain goes up on a new act. It is a moment of real national renewal and change.
"This is the dawn of a new era in which we no longer accept that your life chances – your family’s life chances – should depend on which part of the country you grow up in.
“This is the moment we begin to unite and level up.” The PM refused to allow broadcasters to record his address and used his own crew.
