The two men were from Students for a Free Tibet, said Lhadon Tethong, the New York-based group's executive director.
The other two who were detained, a man and a woman, provided support at the base of the lampposts, she said.
Firefighters brought the protesters down and police led them all away peacefully after about an hour.
"At this moment the Tibetans inside Tibet are brutally and violently being crushed by Chinese authorities," Tethong said.
"It's absolutely critical that ... a message is sent to the Chinese government to meaningfully address and end violence and repression in Tibet or they will never be truly accepted by the international community."
The group identified the protesters as Britons Iain Thom, 24, and Lucy Marion, 23, and Americans Phill Bartell, 34, and Tirian Mink, 32.
The National stadium, also known the Bird's Nest, will host the Olympics opening ceremony on Friday and is ringed with metal fences and tight security, including China's latest anti-aircraft missiles.
|The protest marred the final leg of the Olympic torch relay [EPA]
The incident marred Wednesday's carefully stage-managed torch relay ceremony which began early in the morning in the Forbidden City, the former palace of China's emperors, with Yang Liwei, China's first astronaut, kicking it off.
Tibet has been an extremely sensitive topic since protests against almost 50 years of Chinese rule turned violent in the region's capital of Lhasa in March.
Many Tibetans say they were an independent nation before communist troops invaded in 1950, while Beijing says Tibet has been part of its territory for centuries.
Similar demonstrations were sparked in Tibetan communities throughout western China and a massive crackdown by Chinese security forces ensued.
The uprising brought criticism from abroad and the Olympics torch relay provided a lightning rod for protests around the world.