Rights activists began a legal bid on Monday to compel South Africa to investigate and prosecute citizens of neighbouring Zimbabwe suspected of crimes against humanity.The activists argue that South Africa is failing to meet its obligations in international law. A court ruling in their favour would cause a headache for South Africa, which could see its courts clogged with prosecutions and its diplomacy with the power-sharing government in Harare hobbled.President Robert Mugabe and top members of his ZANU-PF party are already subject to international sanctions for suspected human rights abuses including using death squads to intimidate voters and torturing political prisoners. Zimbabwe’s courts and security apparatus are largely under ZANU-PF control."The High Court has an opportunity to set an important precedent, which will ensure that South Africa lives up to its legal responsibilities to prosecute the perpetrators of international crime,” said Nicole Fritz, executive director of the Southern Africa Litigation Centre, one of the applicants who brought the case to court.Prosecutors were not immediately available for comment but have said they are looking into the matter. Legal experts say it is questionable whether South African courts could be used to hear cases of crimes committed in Zimbabwe.Last week a court in Zimbabwe found six activists guilty of trying to topple Mugabe for meeting to watch videos of the Arab Spring protests that toppled northern African leaders, but sentenced them to fines and community service instead of the 10 years’ jail demanded by prosecutors.Mugabe, in power for more than 30 years, says the allegations of abuses are part of an international plot to unseat him.South Africa allowed more than a million people from Zimbabwe to enter without documents three years ago when its destitute neighbour was swept up in political violence and its already unsteady economy was being crushed by hyperinflation.Many thousands of these say they are victims of political violence, and so could bring lawsuits if the court decides in favour of the rights groups.