Rachel Johnson has insisted that standing as a candidate for pro-Remain party Change UK is “not a vote against” her brother Boris.
The writer is aiming to become a member of the European Parliament (MEP) for Change UK – The Independent Group in the South West region.
She was joined on stage at We the Curious, a science museum in Bristol, by the party’s MPs and other candidates.
Those chosen from 3,700 applicants include candidates from “every background”, interim leader Heidi Allen told the event.
Former BBC Newsnight presenter Gavin Esler, who has more than 40 years’ experience in journalism, will stand in London.
Speaking after the launch, Ms Johnson said her move was not a vote against her pro-Brexit brother and Conservative Party MP Boris Johnson.
“I’m sure that Boris understands why this is not a vote against Boris,” she said.
“This is a vote for change. We need to move the dial. People need to have a say.”
The party was formed nine weeks ago, when eight Labour MPs and three Tories left their parties.
Ms Allen, MP for South Cambridgeshire, described it as a “pro-Remain alliance”.
“We are here today with a very strong message,” she told the event.
“If you are fed up with our broken politics, join us, get involved and let’s change it together.”
European parliamentary elections are due to take place on May 23 if Britain is still a member of the EU.
Ms Allen said: “We are here to say we stand ready.
“These elections are a chance to send the clearest possible message.
“We demand a People’s Vote and the right to campaign to remain in the European Union.
“We are not afraid to say it as clearly as that.”
She said people “from every corner of the UK and every background” offered to stand as candidates for the party.
“I would like to thank every single one of them today for standing up and being prepared to be counted when their country needed them the most,” Ms Allen told the event.
Chosen candidates come from the public and private sectors, from those with no prior political affiliation to others who have changed party.
“This is no rebel alliance,” Ms Allen said.
“This is the home of the Remain alliance.”
Mr Esler condemned Nigel Farage, leader of the Brexit Party, and Jacob Rees-Mogg, Tory MP for North East Somerset, as “pretend men of the people”.
He said they were the “posers of Brexit” who were “selling the same old snake oil” and he was “sick of it”.
“They claim to speak for the people – they do not,” Mr Esler said.
“They stole our patriotism and I want it back.”
The journalist said he had never been a member of a political party before but was “seriously worried” about the future of Britain.
“Our political system is a joke. It is a worldwide joke,” he told the event.
“They are laughing at us – not with us, at us. It is broken. This country cannot possibly be strong abroad when it is weak at home.
“We all know any version of Brexit will make us poorer as a country.”
Chuka Umunna, MP for Streatham, described Mr Farage as “a professional politician who entered politics three decades ago”.
“He seeks to dress up his brand of xenophobic nationalism as change today,” Mr Umunna said.
“He’s changed the name and the party colour but it’s the same old nasty politics.
“This isn’t change because we’ve heard it all before. You can either bow down to this nonsense or you can challenge it.”
Other candidates standing for the party include former Tory MP Neil Carmichael, former Liberal Democrat MEP Diana Wallace and former Labour MP Jon Owen Jones.
The venue for the party’s launch changed its name from At-Bristol to We the Curious in September 2017.