Zimbabwe is losing at least US$1 billion annually to corruption, with police and local government officials among the worst offenders, Transparency International said in a report.

Social media groups like #ThisFlag and #Tajamuka have cited corruption in President Robert Mugabe's government and police roadblocks where money is taken from motorists as among the main reasons for protests that have rocked the southern African nation in the last few months.

Transparency International Zimbabwe (TIZ) said the police, local councils, the vehicle inspection department that issues driving licences and the education department were among the most corrupt institutions.

"The resulting institutionalisation and systematisation of corruption in Zimbabwean political and economic spheres has been extensive," TIZ said.

"It would be surprising if the value (of corruption) were less than US$1 billion annually."

Critics and the opposition accuse Mugabe of failing to tackle high-level graft and say endemic corruption is one reason foreign companies are hesitant to invest in the country.

Mugabe has at times admitted to corruption among his cabinet ministers but says police lack the evidence to prosecute.

"It could be true there could be corruption but we don't have people who are prepared to give evidence," Information Minister Christopher Mushohwe said in response to questions about the report.

"Give us the evidence and the law will take its course."

Zimbabwe was last year ranked 150th out of 168 countries on the Transparency International index, which measures public perceptions of corruption in public institutions.

Zimbabwe's tax authority in May suspended its head and five managers in connection with the purchase of luxury cars that were undervalued by a local dealer, one of few high-ranking graft cases to be made public in recent years.

On another development, Zimbabwe's High Court upheld a month-long police ban on protests in the capital on the grounds that this was necessary to preserve peace, a lawyer representing opposition activists said.

Police banned public demonstrations until Oct 15 following anti-government protests that led to violent clashes with police.

Tendai Biti, a former finance minister and prominent lawyer, said Judge President George Chiweshe had in his ruling dismissed the application to overturn the police ban.

Biti said the judge, while accepting that the law used by the police violated individual rights allowing peaceful demonstrations, it was nonetheless justified in order to protect property and ensure public peace.

Biti said he would meet other human rights lawyers on Wednesday to decide whether to appeal Chiweshe's ruling at the Supreme Court or launch a challenge at the Constitutional Court.

Angered by a jobless rate above 80 percent, corruption and the worst fiscal crunch and cash shortages since the adoption of the U.S. dollar in 2009, many Zimbabweans have taken to social media to organise anti-government activities.