Voting to leave the European Union will only benefit a national oligarchy who are keen on ruling over British people "completely and utterly undemocratically", says economist Yanis Varoufakis.
Speaking at a rally called Another Europe Is Possible, held at London's Institute of Education on Saturday, the former Greek finance minister said both the Remain campaign and Brexiters are "infantilising voters in a cynical and astonishing fashion".
In a mix of left-wing political perspectives, Mr Varoufakis joined Green Party MP Caroline Lucas, Labour MP Clive Lewis, and journalist Owen Jones, amongst others, to make their case for remaining in but overhauling the current European Union.
Mr Varoufakis said that, as people who come from different political background and parties, they are standing united, "working and campaigning together" to achieve the change they crave across the continent.
"It is impossible for the European Union to become democratised without democratic Britain participating in the process," said Mr Varoufakis.
"Voting to leave the European Union will only benefit a national oligarchy and class which is particularly keen on ruling over the British people completely and utterly undemocratically."
Referring to the issue of migration, often brought up in EU debates, he told the crowd he agreed that public services in certain parts of the UK were "strained" because of this.
He said it left many with the feeling that they have been "marginalised in their own country".
"This feeling is not caused by migration, it is merely correlated with it. The reason public services are failing is the running austerity feed that promotes a vicious class war against Britain's poor," he added.
"Without the labour, skills and dedication of migrants staffing the National Health Service and other public services, those public services would have collapsed already."
The event, organised by Open Democracy, brought together hundreds of left-wing supporters in an attempt to form an action plan ahead of the June 23 vote.
Ms Lucas, MP for Brighton Pavilion, told the crowd that another Europe is possible and that those on the left have been "too quiet" for too long over their support for the union.
"Europe needs to be more democratic, accountable and transparent - but so does Westminster," she added. "We need to be in it to change it."
Ms Lucas said that issues such as climate change affect all EU countries and "do not wait at borders for their passports to be checked".
She added: "This is not just about how much we put in and take out, it goes to the heart of the kind of country we want to be.
"To what kind of people we are and the future we want."
Labour's shadow Chancellor, John McDonnell, was due to speak at the event later.