Magistrate Catherine Chimanda granted the activists, including human rights campaigner Jestina Mukoko, bail of $600 on condition they surrender their passports and report to police once a week.

Their indictment and imprisonment brought new tension in the power-sharing government of President Robert Mugabe and Prime Minister Morgan Tsvangirai.

The activists were ordered back to jail on Tuesday, drawing condemnation from Western countries and human rights groups.

The activists, who also include several members of Tsvangirai’s Movement for Democratic Change (MDC), say they were abducted by state security agents from their homes last year and tortured to force them to confess to planning to remove Mugabe from power.

The activists were granted bail in March with the consent of state prosecutors.

Human Rights Watch said in a statement Zimbabwean authorities should drop the criminal charges against the activists.

"This continued persecution makes it pretty clear that (Mugabe’s) ZANU-PF (party) is trying to undermine the new power-sharing administration and is an example of Zimbabwe’s overall lack of progress in respecting the rule of law and basic rights," said Georgette Gagnon, HRW Africa director. 

Long-time rivals Mugabe and Tsvangirai formed a unity government in February after months of wrangling but sharp differences 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 jailed for a month in February on charges of plotting terrorism.

Western donors have demanded the unity government carry out wider political and media reforms and called for the release of all political prisoners before committing funding.

The United States said it was troubling that the activists were jailed again while Britain said on Tuesday the imprisonment of the activists were disappointing.

"The new government in Zimbabwe has made some notable progress since its inception in February. This makes news of the return to custody of 18 high profile political detainees all the more disappointing," British Foreign Secretary David Miliband said in a statement.