The Arab Organisation for Human Rights in Syria said in a statement on Saturday that the six were arrested on 12 May and alleged that authorities arrested all the Islamists who took advantage of an unofficial amnesty to return home.
The organisation said Montasser al-Naeb was arrested in northern Latakia province. It accused Syrian authorities of torturing al-Naeb, who was charged with belonging to the banned Muslim Brotherhood.
It accused Syrian authorities of arresting all exiled Islamists who return home.
The other five, Hussein Mohammad Marei, Tarek al-Zein, Firas al-Khouli, Ahmad Awad and Bilal al-Tatari, were also arrested on the same charges in Zabadani, a city on the outskirts of Damascus, the organization said.
There was no official comment on the group's report. Syrian authorities rarely comment on security matters.
Though Syria has not formally declared an amnesty, Syrian diplomatic missions abroad were instructed last month to give passports to Syrians wishing to return home.
Human rights activists have said dozens of exiled Syrians have taken up the offer, but some were arrested on arrival.
The activists have warned the exiles not to return without guarantees for their safety or until a general amnesty for all political crimes is issued.
"It seems that practicing religious rituals has become an obsession for all government security agencies," the organisation said.
The Syrian government is urged "not to harm nationals who are practicing their religion"
Arab Organisation for Human Rights in Syria
It urged the government to release all detainees and "not to harm nationals who are practicing their religion."
President Bashar Assad has released hundreds of political detainees since coming to power in July 2000.
But he has also clamped down on pro-democracy activists, showing there are limits to dissent his administration is willing to tolerate.