The activists, including leading human rights activist Jestina Mukoko, say they were abducted by state security agents from their homes last year and tortured to force them to confess to planning to remove President Robert Mugabe from power.
The activists, who also include several Movement for Democratic Change (MDC) members, were granted bail in March with the consent of state prosecutors.
"We were surprised by the magistrate’s decision as we were making prior arrangements with the state. She just said the matter was now outside her jurisdiction and remanded them in custody," Alec Muchadehama, one of the defence lawyers, said.
Muchadehama said the 18 had been indicted for a trial which starts next month. Lawyers were preparing to apply for bail at Zimbabwe’s High Court later on Tuesday.
Long-time rivals Mugabe and Prime Minister Morgan Tsvangirai of the MDC formed a unity government in February after months of wrangling but sharp differences still remain over issues such as the review of the posts of central bank governor and attorney general.
Mugabe has yet to swear in Roy Bennett, a senior white MDC member, as deputy Agriculture Minister. Bennett was locked up in prison for a month in February on charges of plotting terrorism.
Western donors, who have demanded that the unity government carry out wider political and media reforms and called for the release of all political prisoners before committing funding, are likely to raise concern.
"As the MDC we are very concerned with this matter as it adds to a long litany of breaches to the Global Political Agreement (signed by Mugabe and Tsvangirai on September 15)," an MDC official told Reuters.