Crowds of Bernie Sanders-supporting US expats rallied in London's Parliament Square on Tuesday evening before making their way to the city's Democrats Abroad polling station.

Members of London For Bernie, an organisation set up to unite Democrats living in the UK backing Bernie Sanders's presidential campaign, congregated in the centre of London's political district carrying banners and signs with messages of support for Hillary Clinton's closest rival.

Democrats gathered outside Parliament carrying signs and banners Adam Payne/Business Insider The group says it has over 800 registered supporters based in the London area alone, and turned out in mass to take the candidates' message to the streets before casting their vote from across the Atlantic.

"Super Tuesday" doesn't just mark the day where twelve US states cast their vote; it also represents an opportunity for the many Democrats living abroad to have a say in who enters the race to the White House.

Anya, 18, an international-relations student, was in Parliament Square to declare her support for Sanders, who she believes is the candidate to lead the party into the upcoming election.

Anya, originally from New Jersey, said:

Bernie resonates so much abroad because of his domestic policies and certainly his foreign policy — he really wants to put the US back in its place where it should be. He speaks the truth and his message is strong and clear. He isn't shaded by the pressure of Super PACs and big corporations.

Derek, originally from California, studies with Anya. He said Sanders has his vote because he is a politician with principles:

What struck me at first was [Sanders'] consistency. It's very common in politics for people to adjust their opinions along with what's popular or what will get them elected. I've watched old clips of Sanders and the things he was saying then are the things he is saying now.

He added: "If we had heeded what he had said about finance reform and the Iraq war then the nation would be in a much better position now."

The crowd chanted "feel the Bern, feel the Bern!" Adam Payne/Business Insider

The Democrats Abroad Global Primary got underway on Tuesday evening and will be open until March 8, with voting centres in the UK at London, Oxford, Cambridge, Edinburgh, and St. Andrews, as well as 40 other countries worldwide.

Bill Bernard, a former chair of Democrats Abroad UK, has been involved in electoral politics for decades and said the Democrat turnout in Britain is as big and healthy as it's ever been.

The polling station was based at the Abbey Centre, South London Adam Payne/Business Insider Speaking to Business Insider UK, Bernard said:

It's a great occasion. We had a similar turnout in 2008 when it was Hillary Clinton versus Obama. The turnout was enormous that year and today seems to be equalling that.

A long queue of US expats awaited volunteers at the polling station at the Abbey Centre, a conference centre a short walk from the Houses of Parliament — an indication of just how much interest this year's Democratic primary has attracted.

Bernard said:

It's really not that unusual that a candidate from the left is challenging a moderate, centrist Democrat. What's incredibly unusual is what's happening on the Republican side. I've been involved in politics for a long time and I have never seen anything like this.

Democrats Abroad will send a total of 21 delegates to the Democratic National Convention in July.

Here's video of Sanders' fans chanting in support of his candidacy: