Zachary Katznelson, a lawyer from the British human rights group Reprieve, said bin Omar, who had been held without charge since August 2002, faced "grave risk" of abuse and torture in Tunisia for his involvement with Ennahdaha, which he described as a non-violent Islamic political party.
"I hope and pray Tunisia is going to do the right thing but I don't know that we can rely on that," Katznelson said.
He said Reprieve tried without success to persuade the US to halt or delay bin Omar's transfer after his family said he had been convicted in absentia and sentenced to 23 years in prison for his involvement with the banned political group.
Bin Omar, who is married and has eight children, fled Tunisia to avoid political persecution, according to Reprieve, and unsuccessfully sought political asylum in Pakistan, where he was living when he was captured by US forces.
'Credible assurances'
Navy Commander Jeffrey Gordon, a US military spokesman, said no detainees were transferred out of Guantanamo without "credible assurances" from their government that they would be treated humanely.
Obtaining those assurances from Yemen – which has the most detainees held at Guantanamo, about 100 – has been difficult, which makes Tuesday's announcement that four of the detainees were sent to that country relatively rare.
Since Guantanamo opened in 2002, the US has released about 405 prisoners after determining they were not a threat, did not have any intelligence value and their home country would be capable of preventing them from "rejoining the fight" against the United States or its allies.
Many of those transferred to the custody of their native countries have been released.