The campaign has raised over £300,000, which will be spent organising tactical voting and supporting candidates opposed to a "hard Brexit".
Last year Ms Miller fronted the legal challenge that forced the government to consult Parliament before formally beginning the Brexit process.
She said MPs had to be "open-minded about what is best for Britain".
Speaking at PMQs earlier, Prime Minister Theresa May said a vote for the Conservatives would strengthen her negotiating hand to secure the best Brexit deal for Britain.
But at the launch event for her Best for Britain campaign, Ms Miller - who will not be standing for election herself - said people needed to vote tactically on 8 June to avoid an "elected dictatorship" and a hard Brexit.
She dismissed suggestions she was too divisive a figure to front the initiative, saying it was a non-party political movement that wanted the best outcome for Britain and a range of voices in Parliament.
The campaign said it was still "analysing data and algorithms", but could intervene in up to 100 marginal seats.
But outgoing former UKIP MP Douglas Carswell predicted the campaign would actually secure extra votes for the Conservatives over Labour in key marginal seats.
Businesswoman Gina Miller has defended her crowdfunded bid to keep "all options on the table" on Brexit.