The bodies of four opposition activists have been found near the Zimbabwe capital, after being abducted, the Movement for Democratic Change says.
An MDC spokesman accused supporters of President Robert Mugabe of being behind the deaths, ahead of next week's poll.
The body of Harare's recently elected opposition mayor's wife has also reportedly been found, badly burnt.
Tanzania, current chair of the African Union, says there is every sign the election will not be free and fair.
Tanzanian Foreign Minister Bernard Membe said he and his counterparts from Angola and Swaziland would be appealing to their own presidents to take urgent action "so that they do something urgently so that we can save Zimbabwe".