Hundreds of thousands of pro-Europeans from across Britain were expected to march through London on Saturday calling for another referendum on EU membership with the country mired in political paralysis over Brexit.
Opponents of Britain's departure from the European Union will gather near Hyde Park from noon (1200 GMT) before converging on Westminster in what organisers are calling the "Put it to the people march".
Speakers including Scotland's First Minister Nicola Sturgeon, London Mayor Sadiq Khan and opposition Labour deputy leader Tom Watson will then address the crowds at a mass rally outside parliament.
"Brexit is a complete and utter mess," Khan said on the eve of the event.
"I'll be marching on Saturday with people from every part of our country -- from every walk of life -- to demand that the British people get the final say." The protest -- set to be one of the largest in the capital in decades -- comes after EU leaders this week granted a delay to Brexit, prompting Prime Minister Theresa May to make a renewed bid to win MPs' backing for her divorce deal.
However she faces daunting odds with lawmakers deadlocked for months over how to implement the 2016 referendum vote to leave, reflecting bitter divisions nationwide.
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If she succeeds, Britain -- which was staring at a cliff-edge deadline of March 29 for leaving the EU -- will depart on May 22 under the terms of the withdrawal agreement the prime minister struck with Brussels last year.
But if lawmakers defeat the accord again, as expected, London must outline a new plan or face a no-deal Brexit as early as April 12 -- unless it decides to request another extension and hold European Parliament elections in May.
Any further delay would likely prompt further calls for another referendum as the only way out of the impasse.
Hosted by the People's Vote pressure group, Saturday's protest follows a similar demonstration in October that drew an estimated half a million people.
Organisers have arranged hundreds of coaches and even chartered a train to bring protesters from all corners of the country to the capital.
Among those planning to attend: Stephen Goodall, a 96-year-old World War II veteran, who will travel 200 miles (300 kilometres) by train from southwest England alongside four generations of his family including his great-granddaughter.
"I am an old man and the outcome won't affect me -- but it will affect my family and many people that I know for years to come," he said in a statement released by organisers.
The prime minister has repeatedly ruled out holding another poll on the issue, claiming it would be divisive and renege on promises to honour the 2016 referendum result.
Meanwhile the main opposition Labour Party appears divided on the issue.
At its 2018 conference, it backed holding another poll as a last resort, while advocating staying in a customs union with the EU together with close alignment with its single market.
But some MPs are fierce advocates of putting it back to the people, while others representing Leave-supporting areas in central and northern England, are bitterly opposed.
In a sign of the splits, at least half a dozen Labour shadow ministers are set to join deputy leader Tom Watson at Saturday's march, while the party itself asked activists instead to help campaign for local elections due on May 2.
Watson said he had now decided to campaign for a referendum "reluctantly" and would back May's deal if it was also put to the people.
"It can only begin to bring the country back together again if we all have a final say -- and then live with the result," he said.
"I trust the people I represent. And only they can sort this mess out.