About 50 people gathered outside the South African High Commission, the BBC reported. They chanted "Zuma save Zimbabwe."
During discussions with British leaders, Zuma argued in favor of relaxing sanctions against Zimbabwe. The protesters accused him of favoring President Robert Mugabe, who has ruled Zimbabwe since 1980.
Some of the protesters also attacked Zuma as a polygamist. He has three wives and has fathered 20 children.
Prime Minister Gordon Brown and Zuma discussed Zimbabwe and held a joint news conference after their meeting. Brown said the European Union should not relax sanctions until Zimbabwe has guaranteed human rights for its citizens.
"I think there has been a greater understanding of what we are trying to do in Zimbabwe and what are the concerns," Zuma said.
Zuma's two predecessors, Nelson Mandela and Thabo Mbeki, also made state visits to Britain.