The group, which includes Jestina Mukoko, a leading human rights activist, and several MDC members, were granted bail in March with the consent of state prosecutors.
The 18 have now been indicted for a trial which starts next month.
'Shadow over government'
Fifteen of the activists appeared in court on Tuesday for the remand hearing, the other three are still in hospital recovering from injuries they said they received at the hands of security forces.
The judge at the hearing said that the activists' bail had been revoked because prosecutors had filed a formal indictment.
Alec Muchadehama, one of the defence lawyers, said: "We were surprised by the magistrate's decision as we were making prior arrangements with the state. She [the judge] just said the matter was now outside her jurisdiction and remanded them in custody."
The move has raised fears for the future of Zimbabwe's nascent unity administration.
Mugabe and Tsvangirai formed a unity government in February after months of political wrangling, but sharp differences remain.
Mugabe has yet to swear in Roy Bennett, a senior white MDC member, as deputy agriculture minister.
Bennett was also jailed for a month in February on charges of plotting "terrorism".
In New York, rights group Amnesty International said the activists' detention "casts a shadow over the new government and calls into question its commitment to halt persecutions of political opponents".