The activists - three men and a woman from Students for a Free Tibet - entered China on tourist visas. Two of them were to leave China on Wednesday, with the other two to follow Thursday.
At dawn they placed two banners calling for Tibetan independence on a lamp post outside Beijing National Stadium, which will host the Games' opening ceremony Friday. One read "One World, One Dream, Free Tibet," the other called for a "Free Tibet" in English and Chinese.
Pro-Tibet demonstrations have intensified in the run-up to the Beijing Olympics, while China has repeatedly called for the Games to not be politicized.
The Olympic torch relay was met by pro-Tibetan demonstrators in cities around the world, including in Paris and London, following unrest in early March in Tibet that left 19 people dead and 623 injured.
