In daring stunts, two men scaled the 52-storey New York Times building within a matter of hours in the presence of hundreds of on-lookers.

The first man, Alain Robert, a Frenchman who has earned nickname of 'spider man' for his audacious climbs including Eiffel Tower [Images] in Paris, was seeking to draw attention to global warming through his death-defying act.

Robert went up the 748-foot skyscraper and unfurled a banner reading, 'Global Warming Kills More People than 9/11 Every Week', but was arrested as he stepped out at roof.

As he was being taken away, another man, identified as Renaldo Clarke began climbing the building to draw attention to childhood malaria. He was wearing orange pants and white T-shirt carrying the message, 'Malaria no more, save the children'.

Both climbed the latticework facade of the year old building without ropes or safety harness as news helicopters broadcast the feat live and people below shot their pictures with cameras and cells phones.

"He was staring at me on the fourth floor," said Kim Severson, a reporter for The Times' Dining section, who saw Clarke pass-by.

"At first, I thought, 'Is he a window washer?' But he had no equipment. He turned and climbed up at a very rapid pace. He looked very focused, solemn and determined."

But witnesses said Robert seemed better prepared with a fanny pack that contained chalk to help his grip and liquid to drink.