It called on neighbouring African states to intervene to prevent bloodshed.
"I say to my brothers and sisters across the continent – don't wait for dead bodies in the streets of Harare," Tendai Biti, the MDC secretary-general, said.
"Militias are being rearmed, Zanu-PF supporters are being rearmed…. The long and short of it is that there has been a complete militarisation of Zimbabwean society since the 29th of March 2008," he added.
A farmers' union said independence war veterans, used as political shock troops by Mugabe, had evicted mostly white farmers from their land since the weekend.
Trevor Gifford, the president of the Commercial Farmers Union, said there was "a lot of trouble and lawlessness out there".
"Farmers are being forced out. In the last three days, we are looking at about 60 who have been evicted," he said.
Zimbabwe state television reported on Tuesday night that war veterans had occupied 11 farms in the northeastern part of the country.
Police said they were not aware of the farm invasions.
The veterans have already spearheaded the eviction of most white farmers under a controversial Mugabe land redistribution programme designed to redress injustices dating back to the British colonial era.
The MDC said Mugabe, 84, was trying to buy time to prepare for a run-off against Morgan Tsvangirai, the MDC leader whom the party claims won the presidential poll outright.
Tsvangirai wrote in a newspaper article that Zimbabwe was on a "razor's edge" because of Mugabe's efforts to cling to power.
Zimbabwe's election law requires a presidential run-off to be held three weeks after official results are announced if there is no winner with a sufficient majority.
The winner requires 50 per cent to win the polls outright and Tsvangirai's party says he won 50.3 per cent of the vote.
The opposition warning comes amid growing frustration over the delay in releasing the official results of the country's elections that ended 10 days ago.
The hearing in a Harare court of an opposition request calling for the immediate release of the results has entered its fourth day, with no end in sight.
Archbishop Desmond Tutu, speaking in the US, said Mugabe could still redeem himself by stepping down as president to ease post-election tensions.
"In a way we need to hang our heads in shame," he said. "I never imagined a day would come when I would have to excoriate him."
The South African Nobel peace laureate added that international peacekeeping troops may be needed to restore order in Zimbabwe.
In another legal case complicating the election stalemate, police said at least five poll officials around the country were due to be charged with undercounting votes cast for Mugabe.