Atheist buses have taken to the streets of London with adverts questioning the existence of God, in answer to posters put up by religious groups.
Campaign organisers had at first decided to put the adverts on just a few buses to counteract the posters the religious groups had put up, in which non-believers were "threatened eternal damnation".
The fundraising campaign's target had been just 5,500 pounds, but they far exceeded it when prominent atheist Prof Richard Dawkins and the British Humanist Association added their support, and they ended up collecting more than 140,000 pounds.
With the campaign money exceeding the target, there was enough money to put the retort, "There's probably no God. Now stop worrying and enjoy your life", on 200 buses, plying in the capital, for a month.
Another 600 buses have been added, and passengers and passers-by in cities across England, Wales and Scotland, from Aberdeen and Dundee to York, Coventry, Swansea and Bristol, will see them.
The atheist bus adverts had been thought by writer Ariane Sherine, who believes that they will brighten people's lives.
"You wait ages for an atheist bus, then 800 come along at once. I hope they will brighten people's days and make them smile on their way to work," the Telegraph quoted Sherine as saying.
The buses have also been accepted by religious groups, who see them as a means of increasing the debate about faith.
Paul Woolley, director of Theos, a theology think tank that donated 50 pounds to the cause, has welcomed the ads.
"The posters will encourage people to consider the most important question we will ever face in our lives," he said.
Though some atheist supporters of the campaign were disappointed that the ads did not declare categorically that God does not exist.
Prof Dawkins, renowned evolutionary biologist and author of The God Delusion, had wanted something more prominent.
"I wanted something stronger but with hindsight I think it's probably a good thing because it makes people think. It's just food for thought - people will have conversations in pubs when they see these buses," he said.
Hanne Stinson, chief executive of the British Humanist Association, said that the adverts were "overwhelmingly positive", and were intended to reassure agnostics and atheists that there is nothing wrong with not believing in God.