Crowds join People's Vote march for final say on Brexit deal

London Mayor Sadiq Khan, front center, holds a klaxon horn as he joins protesters in the People&#39;s Vote March for the Future on Saturday.

London (CNN)Huge crowds marched through central London on Saturday demanding a referendum on the terms of the United Kingdom's exit from the European Union next year.

Organizers of the event, dubbed the People's Vote March for the Future, estimated that more than half a million people were taking part. CNN could not independently verify the number of marchers.
A spokesperson for People's Vote said the crowd stretched three-quarters of a mile through the streets and represented the biggest march in London since the 2003 protest against the war in Iraq.
    Some chanted "People's vote! People's vote!" as they marched, while others shouted "Hey hey Theresa May -- immigrants are here to stay" and "Exit Brexit."
    Organizers handed out postcards for people to write to their local lawmakers demanding a final say on the Brexit deal, which remains under negotiation between the UK government, led by Prime Minister Theresa May, and the European Union.
    The march was led by young voters, some of whom were not eligible to vote in the referendum as they were under 18.
    A British Union flag is held aloft during the march through London on Saturday.