The UK protesters are Iain Thom and Lucy Fairbrother. The campaign group said Mr Thom, 24, is from Edinburgh and Ms Fairbrother, 23, is from Cambridge.
The protest happened before the Olympic torch's arrival in Tiananmen Square. The British Embassy is investigating.
'Critical time'
Students for a Free Tibet named the American protesters on their website as Phill Bartell, 34, from New Jersey and Tirian Mink, 32, from Portland, Oregon. The group added that Lucy Fairbrother is also known as Lucy Marion.
The protesters scaled a 120ft (36.5m) lighting pole early in the morning and unfurled banners reading "One World One Dream Free Tibet" and "Tibet will be free".
Speaking on a mobile telephone, while suspended from the pole, Mr Thom told BBC News: "I'm here today because I've been a long-term Tibet activist and I feel like now is a really critical time for Tibet.
"The Beijing Games have been used by the Chinese government as a propaganda tool to whitewash their human rights record in Tibet."
We have taken non-violent action at this critical time to draw the world's attention to the crisis gripping Tibet
Lhadon Tethong
Students for a Free Tibet
Clandestine Olympic protests
A British Embassy spokesman said: "We are aware of reports of two British nationals being detained."
"We are in touch with the Chinese authorities and are requesting immediate consular access should this information be correct," he added.
Lhadon Tethong, executive director of Students for a Free Tibet, described the protesters as "brave young activists".
She said: "At this very moment, Tibetans are facing the most severe and violent repression they have seen in decades at the hands of the Chinese government, and we have taken non-violent action at this critical time to draw the world's attention to the crisis gripping Tibet."
Dr Kerry Brown, an expert on Chinese issues with London-based think tank, Chatham House, said the Chinese authorities would be "assessing the situation" following the protest.
He told BBC News: "They are very nervous, obviously, because of the opening ceremony tomorrow and they don't want to spoil that."
Activist explains the motive behind his protest
But Dr Brown added that if he was "brutally" honest, most Chinese were less concerned about the situation in Tibet than with their country's economy.
The Olympic torch has travelled 87,000 miles (140,000 km) through six continents since leaving Greece in March.
It's journey has been marked by protests about China's human rights record and its policies in Tibet.
During the month in which the torch began its progress, protests flared in Tibet against Chinese rule before snowballing into the worst unrest in Tibet for 20 years.
The Chinese Embassy could not be reached for comment.
original article - http://news.bbc.co.uk/1/hi/uk/7544416.stm
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