"We will no longer participate in the violent illegitimate sham of an election process," Tsvangirai, 56, told reporters at his home, saying he could not ask supporters to cast ballots "when that vote would cost them their lives.
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The opposition chief said Mugabe had "declared war by saying that the bullet has replaced the ballot", referring to the president's earlier threats to fight to keep the opposition out of power.
"We believe an election that reflects the will of the people is impossible," he said, as he appealed to the United Nations, African Union and regional body SADC to "intervene and stop the genocide".
Tsvangirai added he would announce a decision on his next moves on Wednesday -- leaving open the possibility, however slight, that he could change his mind.
UN chief Ban Ki-moon on Sunday called Tsvangirai's decision to quit the June 27 run-off election a "deeply distressing development" and a bad omen for the country's future, his spokesman said.
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