Oppostion leader Morgan Tsvangirai has pulled out of Zimbabwe's presidential run-off election.
In broadcasts monitored in South Africa, Tsvangirai described the election campaign as a "violent, illegimate sham of an election process."
He appealed for the African Union and the United Nations to act to prevent "genocide" in the country.
Tsvangirai made the announcement in Harare Sunday after extensive debate within his Movement for Democratic Change. It came against the backdrop of rising violence and intimidation in the buildup to the election, scheduled to be held on June 27. Dozens of activists have been killed, hundreds injured and thousands forced from their homes.
Militia organised by President Robert Mugabe's ruling Zanu PF are responsible for most of the violence, with army and police officers either participating or turning a blind eye to attacks on opposition supporters.
Tsvangirai has been detained repeatedly while campaigning. His party secretary-general, Tendai Biti, has been charged with treason and remanded in custody until after election day. State media have denied publicity to the opposition.
Tsvangirai won the first round of the presidential election on March 29, but the Zimbabwe Electoral Commission said he did not draw the majority to avoid a run-off.