"The long drawn-out process to reach a political settlement was marked by the perpetration of serious human rights violations and caused untold damage to the rule of law in Zimbabwe," said Pillay in a statement.
"All eyes will be on this new government to see if it can undo that damage," the former South African judge added.
Former opposition leader Morgan Tsvangirai was sworn in as Zimbabwe’s Prime minister on Wednesday.
Tsvangirai, a former political prisoner, will share control of the home affairs ministry that controls the police with President Robert Mugabe.
Pillay highlighted abductions and "unlawful arrests" of lawyers, human rights activists, journalists and political opponents in recent months and noted that police had often failed to respect court orders for their release.
"It is vital that international attention is focused on preventing future violations in the country by ensuring that human rights defenders and independent media are able to carry out their work without being harmed, arrested or harassed," Pillay said.
"I call for the immediate release of all those people currently still being held in unlawful custody," the UN High Commissioner for Human Rights added.
The police force has been accused of widespread human rights abuses on supporters of Tsvangirai’s Movement for Democratic Change party, with 30 activists still detained on charges of attempting to overthrow Mugabe’s government. – AFP