We were being watched , but the man with the ragged beard didn't know it. "Any loose change?" he muttered as a motionless figure looked down from the roof of an apartment building, dark against the evening sky. Another was poised on the top edge of a high tower, as if about to jump.
"Who are they?" said the hustler with alarm as he noticed them for the first time. Two more stared from the turrets of the Hayward Gallery; other mute figures were just visible on the London skyline, standing in the most unlikely and dangerous places. "What are they?" None was moving. "What are they for?"
Good question. For getting exactly that kind of reaction, the artist might say. "They are meant to be unsettling," said Antony Gormley, the Turner Prize-winner who is erecting life-size casts of his own body on the roofs of major buildings around the South Bank.
"Their strangeness makes us reconsider a familiar cityscape, and the way we live in it."
The last of the 31 men should be in place for the opening of a related exhibition at the Hayward Gallery on Thursday.
They are supposed to come down in September, but may have to stay if Event Horizon, as the installation is called, has the same emotional impact as the artist's hugely popular Angel of the North in Gateshead.
His mute men looking out to sea at Crosby Beach on Merseyside were meant to be temporary, until locals decided they loved them.
At 56, the sculptor and OBE is the superstar of a new boom in British public art. Across the country artists are trying to stop us in our tracks - from the 100 painted model cows that appeared on the streets of Edinburgh last year to the baffling yellow lines painted on the Cardiff Bay Barrage in March.
But at his London studio on Friday Gormley admitted to nerves: "We need to see how it works out. It is a completely open question as to how this will be received."
Angel of the North, with wings as wide as those of a jumbo jet, was erected in 1998. The 150,000 visitors it attracts every year have helped to regenerate Gateshead, along with the Baltic and Sage centres.
"The reaction to it is a tribute to the generosity of the people of the North-east in welcoming strangers," said the artist, "but you cannot make a policy out of something like that and expect that just because you place a work of art people will take to it."
There are hundreds more sculptures, installations and events in public spaces now than there were a decade ago, and the Angel has been an inspiration for most of them. Gormley has his doubts: "I would hate for its success to be used as an excuse for making spurious things."
This week the minister for culture, David Lammy, will launch a new group called Art in the Open, aimed at helping artists, councils and developers work together. It is funded by the Arts Council, which is in the middle of spending [pound]1.1bn on the arts over two years. Some of that has gone on Gormley's London men. The cost is not being made public, but with the Hay-ward's accompanying Blind Light show it is thought to have exceeded [pound]1m.
The men herald the grand reopening of the refurbished Royal Festival Hall this summer. Sponsorship comes from the law firm Eversheds, and others, including Land Securities, have lent roof space. But why? What is in it for them?
"Art can make people feel welcome," said Patricia Brown, chief executive of the Central London Partnership, through which some of the country's wealthiest firms support the arts. "It can make the city an enchanting place, somewhere people want to be, and open up a sense of wonder."
Which is lovely. But her members also have very hard-headed reasons for being involved. "Cities are competing for our time. Anyone can go to Zara in Madrid or …