UN-appointed experts Thursday urged Zimbabwe to release three rights activists they said had been tortured in detention, as the country prepares to host a key regional summit.

They named them in a statement as rights activist Namatai Kwekweza, teachers' union leader Robson Chere and Samuel Gwenzi, a former Harare municipal councillor.

They were arrested on July 31 for demonstrating in support of jailed opposition leader Jameson Timba and other activists.

The UN experts said unidentified men had forced them off an aeroplane and held them incommunicado for eight hours.

"The three were reportedly subjected to enforced disappearance, torture and other cruel, inhuman or degrading treatment or punishment, including waterboarding", the statement said.

They were afterwards handed over to police who informed them for the first time of the reason for their arrest: alleged "disorderly conduct" during a demonstration in July for Timba's release.

The UN-mandated experts called the charges "baseless" and the detainees' treatment "inexcusable".

A court was scheduled to rule Friday on whether to release them on bail.

Zimbabwe hosts the heads of state from the 16 countries of the Southern African Development Community (SADC) on Saturday.

Global monitors Human Rights Watch and Amnesty International said Zimbabwe authorities have arrested dozens of people in the weeks leading up to the summit.

"At a time when Zimbabwe is preparing to host the SADC summit, whose values include institutions that are 'democratic, legitimate, and effective', it is unconscionable that these human rights defenders working to strengthen such institutions remain arbitrarily detained," the experts said.

The experts included several "special rapporteurs" -- independent rights specialists who are appointed by the UN rights council but do not speak on behalf of the United Nations.