The Australia-based Fiji Democracy and Freedom Movement has expressed its concern about a former parliamentarian, Peceli Rinakama, who has not been seen since last Friday.
Reports say he was seized by soldiers, but the military and the interim government refuse to comment.
Mr Rinakama's whereabouts have been unknown for six days and the Movement's Usaia Peter Waqatairewa says it has failed to find out what has happened to him.
He says he has grave concerns given the military's actions since the 2006 coup.
"They can't guarantee the safety of the people. This regime has failed in its duty as the government of Fiji under the International Declaration of Human Rights to guarantee the safety of its citizens."
During the recent trial of eight men accused of plotting to kill the interim prime minister, the court was told that Mr Rinakama was gathering ex-military personnel for the cause.
He was not among those made to stand trial but was briefly held in 2007 in connection with the plot.