The New York Times building became the prop of two activists who separately scaled the office building’s side walls yesterday and were arrested by police after reaching the roof, 52 stories up.

Officials said Alain Robert, 46 — who calls himself “Spiderman” — began his harness-free ascent at 11:30 a.m. after holding a news conference. He announced that he would climb a building as an act of environmental activism, calling for “urgent action from world leaders on global warming.”

Nearly five hours later, an information technology worker, Ray Clark, 32, began a climb of the building’s south wall. Friends said he planned to scale the building several weeks ago, and never did, but was inspired by Mr. Robert to do it yesterday.

About a third of the way up, Mr. Robert attached a chartreuse banner to the building that read: “Global Warming Kills More People than 9/11 Every Week.”

Moments after he began, EMS was called to the scene. According to Lieutenant Tim Krumm, officers were stationed on each of the building’s floors, but because of the building’s design, they were able to reach Mr. Robert only once he had completed his climb.

Lieutenant Krumm said: “We told him to relax, and he said he was relaxed.”

Sergeant Michael McGuiness climbed out onto a beam on the building’s roof and secured Mr. Robert with a safety rope and harness. He said several of the building’s tiles had fallen off during the climb.

Mr. Robert is a professional climber from France known for similar ascents of the Sears Tower and the Eiffel Tower.

Mr. Clark’s cause was malaria: He wore a shirt that said “Malaria No More.”

“What he’s done today far outweighs any consequences he may face,” a friend of Mr. Clark’s, Nick Ruzhnikov, said.

A police detective, James Coll, who was among a group of officers prepared to rappel down the building to rescue Mr. Clark, said he noticed that the climber had appeared physically exhausted during the climb and “seemed to welcome our arrival.”

A spokesman for the police department, Deputy Commissioner Paul Browne, said Mr. Robert was charged with reckless endangerment, disorderly conduct, and criminal trespass. He said Mr. Clark would likely face the same charges, depending on the outcome of his psychiatric evaluation at Bellevue Hospital.

Mr. Browne said that police officers were now stationed at the building to help the New York Times Co. with security.

A spokeswoman for the company, Catherine Mathis, said in an e-mail statement that the climbs were “illegal and ill-considered.”

“We are taking steps to prevent future occurrences,” she said.