All - including Pema Yoko, a half-Tibetan woman with Japanese citizenship - were detained by police and plainclothes security agents, Tethong said.
John Ray, a journalist working for the ITV news network, was also detained while covering the protest.
"The Tibetan protesters were in the park, John Ray was running behind them, the police was running behind him," said Bessie Du, a Beijing-based producer for ITV, who watched the incident unfold.
Du also said that police put Ray into a car, despite his efforts to show them his Olympic press accreditation.
"I tried to explain to them that I was a British journalist but they would not even let me take out my identification documents, they were very rough," Ray said.
The demonstration was the largest in a string of brief protests - mostly by foreigners hoping to use the Olympics to draw attention to their causes - throughout Beijing since the games started last week.
Most have had less than five people present and foreign activists have been deported.
Also on Wednesday, a rights group said a Chinese activist who applied for permission to protest against corruption during the Olympics has been taken away by security agents.
Ji Sizun came to Beijing from the southern province of Fujian and wanted to demonstrate in one of three protest zones Chinese officials have designated for the games, Human Rights Watch said in a statement.
Ji wanted to call for "greater participation of Chinese citizens in the political processes, and denounce rampant official corruption and abuses of power," the group said.
He applied at a police station in Deshengmenwai on August 8 - the day the Olympics began - and disappeared three days later, when he went back to check on his application, the statement said.
"Eyewitnesses said Ji entered the police station at around 10.45am on August 11. At 12.15pm, he was escorted out of the building and put into a dark-coloured, unmarked car by several men who appeared to be plainclothes policemen."
In July, the Chinese government said that protests would be allowed during the Olympics in three public parks far away from the main sports venues.
According to Liu Shaowu, the security chief for the Beijing Organising Committee, applications to hold demonstrations must be filed five days in advance and would receive a response at least 48 hours before the requested rally time.
The protests must not harm "national, social and collective interests," he said in comments posted on the organising committee's website.