Lawyers for Jestina Mukoko, who heads the Zimbabwe Peace Project, which documents political violence, had filed contempt of court charges after police refused to release her from detention, as ordered by a High Court Judge last month.
Mukoko — one of Mugabe’s most influential critics — and 15 other mainly opposition activists have been charged with recruiting or attempting to recruit people to overthrow the government, banditry and bombing police stations.
Two others face lesser charges.
The arrest of the activists, condemned by the opposition, has increased doubts over implementation of a power-sharing agreement by Mugabe and Tsvangirai.
Police initially denied holding Mukoko, who was seized by gunmen from her home on December 3, and had treated her case as a kidnapping. It later emerged Mukoko was being held by state security agents and police.
High Court Judge Alphius Chitakunye ruled that Mukoko should remain in custody and refused to uphold an earlier ruling ordering police to investigate and disclose the identities of her kidnappers, and where she had been held.
"The law has completely broken down in Zimbabwe, if a High Court refuses to investigate an admitted kidnapping … what else can we do except maybe to go to another court but the result may only be the same," lawyer Beatrice Mtetwa told reporters after the ruling.
State prosecutors, who had appealed the earlier High Court ruling ordering Mukoko be released, said they were happy with the ruling.