It has been described as “secret” and “shadowy” but the pro-remain campaign Best for Britain, which received £400,000 from billionaire George Soros, has said it is planning a UK-wide tour and a blaze of publicity.

The group has launched a crowdfunding campaign in the wake of a story in the Telegraph, which attacked the funding it had received from Soros’s Open Society Foundations (OSF).

Best for Britain said it launched its fundraising drive “to stand up for free speech, democracy and the right to fight this disastrous Brexit”.

Facebook Twitter Pinterest George Soros. Photograph: Olivier Hoslet/AP

Soros, a former refugee who became one of the world’s foremost currency speculators, made more than £1bn betting against the pound on Black Wednesday in 1992, which forced the British government to pull sterling out of the European exchange rate mechanism.

George Soros's foundation backs campaign to reverse Brexit Read more

Last October, the Hungarian-American gave $18bn (£12.8bn), a vast bulk of his personal wealth, to his OSF. Its stated mission is to financially support civil society groups fighting for democracy, education and against discrimination, but the OSF and Soros have been persistently attacked by autocratic states including Russia and Hungary.

Best for Britain began as a bid to encourage tactical voting ahead of the June 2017 elections, in an attempt to get more pro-remain MPs into the Commons.

The group, which is registered with the Electoral Commission, was founded by Gina Miller, the businesswoman who took the government to the supreme court over Brexit, and who spearheaded Best for Britain’s campaign during the June election. Its chair is Mark Malloch Brown, the former UN deputy secretary general.

Everyone has a right to be heard and if a country cannot change its mind, it ceases to be a democracy Eloise Todd

Another donor who has given significant sums to the group is Clive Cowdery, the insurance businessman who founded the Resolution Foundation thinktank.

Its activities are now run by the chief executive, Eloise Todd, a former global policy director for the One anti-poverty campaign. “Today’s Telegraph article says that our right to free speech in our democracy only applies to some people,” she said.

“Everyone has a right to be heard and if a country cannot change its mind, it ceases to be a democracy. This attack on free speech is deeply worrying and we’re fighting back. We will continue to take funds from any eligible donor who – like us – doesn’t want to live in a poorer, more isolated, worse-off Britain.”

Best for Britain is not the best known of the groups that campaigned for Britain to remain in the EU, but has taken a different tact: to oppose leaving the EU entirely rather than pushing for a softer Brexit.

Soros’s decision to fund the group, rather than the higher-profile Open Britain, appears to be down to a personal connection with Todd through the international development sector.

The group has taken a two-pronged approach: to train remain supporters around the country to continue campaigning in a bid to change leave voters’ minds; and to lobby MPs to vote against the Brexit deal Theresa May secures in Brussels, in the hope of forcing a second referendum or general election.

Best for Britain raised £413,000 in the run-up to the general election, donating the cash to pro-Europe candidates in marginal seats, including Labour’s Tulip Siddiq, the Lib Dems’ Tom Brake and Ed Davey, and the Greens’ Caroline Lucas.

A Best for Britain ad from last summer, ahead of the general election campaign. Photograph: Best for Britain

The group said more than 1 million people had consulted its tactical voting website ahead of 8 June to find pro-remain candidates. It has about 20,000 signed up as members, and 50,000 on its mailing list.

Key supporters include the former Labour minister Andrew Adonis and the former Lib Dem leader Tim Farron, whose ex-press secretary Paul Butters now leads communications for the campaign.

Over the coming months the group is planning a series of training events, rallies and stunts to galvanise remain supporters. Training events have been held in seats that were majority leave, but have remain-supporting MPs, as well as seats with leave-supporting MPs.

The events, called Barnstorms, are geared up to train remain supporters to tackle the concerns of leave voters, by engaging beyond the language of hard economics. So far, events have been held in Doncaster, a majority leave town where local Labour MP Caroline Flint, a remain supporter, has said she feels duty-bound to represent her constituents by voting through Brexit legislation.

“We want to support remain voters in areas like that to help them change opinions,” a group spokesman said.

Other events have been held in Wythenshawe, south Manchester and in Vauxhall, south London, where the Labour MP, Kate Hoey, is a vocal Brexiteer. Future events will include rallies with car manufacturers and trade unions.

Downing Street said it had no qualms about the Soros donations to Best for Britain. “There are many political and campaign groups in this country – that’s entirely right and as you would expect in a democracy,” a spokesman said.

“The prime minister’s position on this matter is clear: the country voted to leave the European Union – that’s what we are going to deliver and there won’t be a second referendum.”