Kate Woznow, the group's campaign director, said on Thursday they were exhausted from the repeated questioning but were not physically mistreated while in detention.
Beijing also deported to Hong Kong the group's executive director, Lhadon Tethong, who had been in the capital blogging about "China's Olympics-related propaganda".
"The world will hear us and support us, and if the world supports us then China will have to listen to us"
Tsetan Ngodup, New Delhi protester
"Even though she knew there was a likelihood she was going to be detained, it still seemed that what she was doing, blogging, isn't illegal," Woznow said. "In most countries it wouldn't cause anyone to bat an eye."
A British colleague was also detained and deported, the Free Tibet group said in a statement.
All eight activists from the US, Britain and Canada were ordered to return home from Hong Kong on Thursday.
China claims Tibet has been its territory for centuries, but many Tibetans say they were self-ruled for most of that period.
Chinese communist troops occupied Tibet in 1951, and Beijing continues to rule the region with a heavy hand.
Meanwhile, about 10,000 Tibetan men, women and monks marched through cities in India and the US on Wednesday shouting slogans and waving flags in protest.
Kalsang Godrukpa, president of the Tibetan Youth Congress, the main organiser of the rally in India, said the essence of the Games was "equality but we do not have equality in Tibet".
"China doesn't deserve the Olympics until Tibet is free," he said in New Delhi.
Tsetan Ngodup, another Delhi protester, said: "The world will hear us and support us, and if the world supports us then China will have to listen to us."
In New York, around 1,000 protesters rallied outside the Chinese Consulate, waving Tibetan flags and carrying signs that read "Boycott Beijing 2008 Olympics" and "No Freedom, No 2008 Games".
Sonam Chonzom, born in India to Tibetan parents, demanded human rights in Tibet, saying, "The Olympic organiser, the IOC, has to know we want human rights. No Olympics in China."