There is probably no God... Now stop worrying and enjoy your life.

A £140,000 advertising campaign aimed at persuading more people to "come out" as atheists was launched today with a plan to broadcast a message doubting God's existence on the sides of buses, the tube and on screens in central London.

Its slogan – "There's probably no God. Now stop worrying and enjoy your life" – can already be seen on buses in central London. A total of 200 bendy buses in London and 600 buses across England, Scotland and Wales will carry the slogan from today and tomorrow following a fundraising drive which raised more than £140,000.

The money raised will also pay for 1,000 advertisements on London Underground from Monday. Organizers today unveiled a set of quotes from famous writers and thinkers who endorse the atheist message.

The launch, held today near the Albert memorial, featured speeches by Dawkins, author of The God Delusion; Ariane Sherine, creator of the Atheist Bus Campaign; and Hanne Stinson, from the British Humanist Association.

The fundraising drive was prompted by a suggestion from comedy writer Sherine, who received support from the British Humanist Association (BHA) and atheist campaigner Richard Dawkins.

Sherine, a television comedy writer, suggested the idea in a Guardian Comment is Free blog last June, saying an atheist bus campaign would provide a reassuring counter-message to religious slogans threatening non-Christians with hell and damnation.

Speaking at the launch , Sherine said the sheer number of donations received had demonstrated the strength of feeling in Britain.

She said: "This is a great day for freedom of speech in Britain and I'm really excited and thrilled that the adverts have been approved and I hope that they will make people smile on their way to work.

"I am very glad that we live in a country where people have the freedom to believe in whatever they want."

There has been overwhelming financial support for the project, which exceeded its funding target less than 24 hours after being launched, raising nearly nine times the amount needed to have its posters on buses.

Around £6,000 was needed to run adverts in London but within two days, individuals and organizations had pledged more than £87,000. More than £135,000 has been pledged so far. Today's event will also reveal the next stages of the campaign.

The theology thinktank Theos welcomed the campaign, saying it was a "great way" to get people thinking about God. "The posters will encourage people to consider the most important question we will ever face in our lives. The slogan itself is a great discussion starter. Telling someone 'there's probably no God' is a bit like telling them they've probably remembered to lock their door. It creates the doubt that they might not have."

The success of the British campaign has inspired atheists in the US to run their own advertising slogans in Washington DC. The American Humanist Association launched a bus advertising campaign last November with the slogan, "Why believe in a god? Just be good for goodness' sake", appearing on the sides, rear and insides of the city's 230 buses.