A man scaled Antony Gormley's Plinth art project in London's Trafalgar Square, minutes before the event was due to begin.
Anti-smoking protester Stuart Holmes took a running jump on to the plinth, but agreed to come down when the first participant was raised into place.
London Mayor Boris Johnson said during his opening speech: "This is brilliant, this is what it's all about."
A new volunteer will take to the plinth every hour for the next 100 days.
More than 14,500 people have so far applied to stand on top of the platform, 22ft (6.7m) above Trafalgar Square.
We are reviewing security in light of what happened
Helen Marriage, event organiser
"I think it's fantastic - it's about capturing art for the people. It's about democratising art," Johnson said.
He told the BBC: "It's a profound meditation on the nature of fame and talent.
"Do these toga'd buffoons cast in bronze around Trafalgar Square deserve any more celebration than this person up there? This is the question - that's what this operation is trying to answer, but I leave it to you."
"You feel very removed from what's going on. It's a lovely view and a view you're never going to see again - it doesn't feel as high as it looks."
Ms Wardell was followed at 1000 BST by Jason Clark, a 41-year-old nurse from Brighton, who took pictures on his camera of the people below.
Other people to take to the plinth on the first day include Jill Gatcum, 51, a consultant from London, Suren Seneviratne, a 22-year-old Sri Lankan student and artist, and Ishvinder Singh Matharu, a 31-year-old optometrist from Chigwell in Essex.
The oldest among July's 615 participants is pensioner Gwynneth Pedler, 83, from Oxford, who plans to signal with semaphore flags.
Heather Pringle, a student from Hexham, will celebrate her 20th birthday on the plinth.
Living statues for following months will be announced later, with 2,400 people taking part up to 14 October.
Participants are chosen at random by a computer and can do anything they want on the plinth - as long as it is legal.
Applications will be accepted until 1 September when the last group of people is chosen.
Watch the first eight hours of the 'Plinth Project' in forty seconds.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.