Zimbabwe's police have defied a court order to release a prominent human rights lawyer detained on Sunday, legal groups have said.
Beatrice Mtetwa's continued detention showed the need to overhaul the police force ahead of elections, they said.
She was arrested with four officials from Prime Minister Morgan Tsvangirai's Movement for Democratic Change (MDC) in the capital Harare.
Zimbabweans voted on a constitution on Saturday to usher in political reforms.
If approved, it will pave the way for presidential and parliamentary elections.
No date has been set for the elections, but they are expected to be held in July.
Mr Tsvangirai, 61, will run against President Robert Mugabe, 89, in the election.
It will herald the end of the coalition government the two leaders formed after the 2007 election, which was marred by violence and allegations of vote-rigging.
Tension has been rising in Zimbabwe ahead of the latest elections.
MDC activist Sten Zvorwadza was assaulted as he tried to put up posters in Harare on Friday, while on Tuesday an official in Mr Mugabe's Zanu-PF party was injured when his house in north-eastern Zimbabwe was attacked with petrol bombs.
Police have charged Ms Mtetwa with obstructing justice after she intervened on behalf of the four officials during a raid on MDC offices on Sunday.
The four are charged with impersonating police officers.
The Zimbabwe Lawyers for Human Rights group, which is representing Ms Mtetwa, said it had obtained a high court order for her release at 02:00 local time [midnight GMT], but she was still in police custody on Monday afternoon.
"The police refused to comply with the order," the group said in a statement.
Four other legal groups - including the International Commission of Jurists and the Pan African Lawyers Union - condemned her detention.
"The arrest of Mtetwa and the four... officials is in itself alarming, but that it comes on the heels of a referendum to endorse a new constitution which, whatever its other limitations, contains strong protection of the rights of those arrested and detained, is more distressing still," they said in a statement.
"Without a clear and unambiguous departure from a past characterised by harassment and intimidation of human rights defenders and of impunity for Zimbabwe's police and security sector, the promise of the new constitution will be laid to waste."
Voters are expected to endorse the constitution, negotiated by Zanu-PF and the MDC.
Both Mr Mugabe and Mr Tsvangirai have called on voters to approve it and for elections to be held in a peaceful climate.
The constitution for the first time sets a two-term limit for the president who will be take office after the election.
It also says the seizure of some 4,500 farms from mainly white people under Mr Mugabe's controversial land reform programme cannot be challenged in court.
The results of the referendum are expected later this week.