The Sadc summit involves leaders from Swaziland, Tanzania and Angola - but not from South Africa, despite earlier indications that President Thabo Mbeki would attend.
His spokesman said he had not been invited. The Angolan representative said Mr Mbeki's views would be taken into account.
Mr Tsvangirai announced he was pulling out of the election on Sunday, saying that government-backed violence against his supporters made a free and fair vote impossible.
The Zimbabwean government has been criticised by the United Nations and South Africa's governing ANC party, among others, for the violence.
The MDC says some 86 of its supporters have been killed and 200,000 forced from their homes by militias loyal to the ruling Zanu-PF party. The government blames the MDC for the violence.
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