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Kate Woznow, of Students for a Free Tibet, said the protest took place today at the headquarters of the state-owned China Central Television building in east Beijing.
She said one Briton and one Australian-Canadian were detained after climbing up part of the building -- a landmark for its unusual shape -- to unfurl the banner.
Wang Wenjie, from the Beijing Public Security Bureau, said he had no information on the protest and sought written questions before he could comment.
Today's protest was the latest in a string of unauthorised demonstrations in Beijing by the student group since just before the Olympics started.