Maria Nyanyiwa, Takunda Gwanda and Nyasha Bosha of the Voice of the People radio were arrested on Thursday and were still being held at Harare's main police station, Thabani Moyo, spokesman for the Crisis in Zimbabwe Coalition, an alliance of rights groups, said on Friday.
"State security agents armed with a search warrant ransacked the offices of Voice of the People and took every piece of equipment in the offices," Moyo told AFP.
In a statement, the rights group accused the government of Robert Mugabe, the Zimbabwean president, of stepping up a campaign of repression.
"The government of Zimbabwe has intensified its internal repression and further moves away from the line of democracy and good governance," the group said.
Immigration authorities have over the past two weeks seized the passports of three critics under new measures to punish perceived enemies of the state.
Authorities returned the passports of Trevor Ncube, a leading independent newspaper publisher, and Paul Temba Nyathi, a prominent opposition official, on Wednesday, but they seized one belonging to Raymond Majongwe, a trade unionist.
VOP, which is based in the Netherlands, broadcasts into Zimbabwe on shortwave. Its offices were firebombed in August 2002.
A rights group says Mugabe has
intensified internal repression
The shortwave radio station is one of only two broadcasters which have managed to circumvent Zimbabwe's repressive media laws by using transmitters outside the country to carry their programmes.
Most of VOP's programming is in Zimbabwe's two local languages, Shona and Ndebele, placing it among the few independent media able to reach the large rural population who have no access to urban newspapers.
Zimbabwe has four radio stations and one television station, all controlled by government.