Zimbabwe's Movement for Democratic Change said on Friday it would not recognise the results of disputed elections that look set to hand President Robert Mugabe a landslide victory.
"We resolved to reject the election and its consequences, which include the government that will result," spokesman Douglas Mwonzora told AFP after a meeting of 13 senior party leaders.
Southern African regional bloc SADC has called on all parties to "respect and accept" the election results in a bid to avoid unrest.
The 15-member body described the vote as "free and peaceful" but stopped short of saying it was fair.
"We reviewed the statement by the SADC observer mission imploring us to accept the results of the election," said Mwonzora.
"We reject that call for the reason that even SADC has failed to endorse the election as fair."
"We cannot be exhorted by SADC to endorse something unfair."
"The national executive will meet tomorrow (Saturday) to chart the way forward for the party and the country."