Mr Mbeki has previously been accused of taking too soft a line with President Mugabe.
Mr Zuma's reluctance to speak out while in London is perhaps understandable, says BBC diplomatic correspondent Bridget Kendall, as it is British hostility which Mr Mugabe claims is the root cause of the problem.
Post-election violence in Zimbabwe has displaced 3,000 people, injured 500 and left 10 dead, according to MDC secretary general Tendai Biti.
Human rights groups say they have found camps where people are being tortured for having voted "the wrong way".
But Zimbabwe's Justice Minister Patrick Chinamasa denies that anyone had died in political violence.
Meanwhile, the state-run Herald newspaper in Zimbabwe has called for the establishment of a transitional government of national unity, under Mr Mugabe.
In an editorial, the Herald says political tensions following the election make it impossible to hold a free and fair run-off in the near future.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.