Jestina Mukoko, head of a local rights group, and the other activists have been charged with recruiting or trying to recruit people to undergo military training to topple President Robert Mugabe’s government.
"The accused cannot be released at this stage, this is a proper case for (a) remand hearing," said Magistrate Mishrod Guvamombe. The activists will appear in court next Monday for a bail hearing.
Their arrests have fuelled political tensions in Zimbabwe, where a protracted deadlock on cabinet posts under a September power-sharing deal has dashed hopes that a new leadership would move to tackle an economic crisis.
Opposition Movement for Democratic Change (MDC) leader Morgan Tsvangirai has threatened to pull his party out of negotiations over the issue.
Two activists facing lesser charges were ordered to be released in line with a High Court ruling last week but state prosecutors said they would appeal the decision.
Thirteen of the activists who will remain in custody are MDC members. Two are colleagues of Mukoko, a former state television broadcaster who has emerged as one of Mugabe’s toughest and most influential critics.
She was taken away at gunpoint at dawn in Harare on December 3 by a group of plain-clothes men who stormed her house and identified themselves as policemen.
A High Court judge last week declared the detention of Mukoko and her eight co-accused unlawful and ordered their immediate release, but the government appealed.
Tsvangirai won the first round of voting in March elections, but fell short of the majority needed to become president, triggering a run-off which Mugabe won after the MDC leader pulled out, citing violent attacks on his supporters.
In their affidavits, the activists say they were severely beaten on the soles of their feet and that they have several scars on their body. They also said they were beaten with fists and blunt objects.
Lawyers have accused the state of appealing the High Court order that they be released to a private hospital to make sure their wounds heal while in custody. Prosecutors say they were unaware of alleged torture.
A cholera outbreak has heightened the humanitarian crisis in Zimbabwe and stepped up pressure on Mugabe from Western and some African leaders who have called on him to step down.
The rate of cholera infections and deaths in Zimbabwe shows no signs of slowing, the World Health Organisation (WHO) said on Tuesday.
It said 1,608 people had died of the disease — which could be treated relatively easily if Zimbabwe’s public sanitation and health systems had not broken down so catastrophically — out of 30,365 reported cases