Hundreds of thousands of people opposed to Britain's withdrawal from the European Union marched through central London on Saturday to demand a new referendum as the deepening Brexit crisis risked sinking Prime Minister Theresa May's premiership.

Marchers set off in central London with banners proclaiming "The best deal is no Brexit" and "We Demand a People's Vote" in what organizers said could be the biggest anti-Brexit protest yet.

After three years of tortuous debate, it is still uncertain how, when or even if Brexit will happen as May tries to plot a way out of the gravest political crisis in at least a generation.