Marchers set off in central London with banners proclaiming “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.
While there was no official estimate of the numbers, campaign organizers said hundreds of thousands of people were in the crowd as it began to march.
In the June 23, 2016 referendum, 17.4 million voters, or 52 percent, backed Brexit while 16.1 million, or 48 percent, backed staying in the bloc.
Some opinion polls have shown a slight shift in favor of remaining in the European Union, but there has yet to be a decisive change in attitudes.
(Production: Paul Hardy, Helena Williams, George Negus, Mussab Al-Khairallah)