Gifford said the veterans forced the farmers to leave their homes with only the clothes they were wearing and that those evicted included at least one black farmer.
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.
Tsvangirai wrote in a newspaper article that Zimbabwe was on a "razor's edge" because of the 84-year-old Mugabe's efforts to cling to power.
Archbishop Desmond Tutu called on Mugabe on Tuesday to accept that he lost the presidential election and step down to ease tensions.
In a speech in San Francisco, the South African Nobel Peace Prize laureate said peacekeepers may be needed to help restore order in Zimbabwe and the country's shattered economy could benefit from a "mini-Marshall Plan" like the initiative which helped rebuild Europe after World War Two.
SPILLOVER
The opposition says Mugabe is delaying the presidential election result to give him more time to prepare for a runoff, and has asked the High Court to force the release of the outcome.
"ZANU-PF is trying to cook the election result in order to engineer and achieve a run-off," Biti said.
The High Court ruled on Tuesday it would treat the opposition's application as urgent and began hearing arguments in the case. It later adjourned until Wednesday.
Legal proceedings are already in their fourth day and could drag further, delaying the end of the stalemate.
Traders in neighbouring South Africa said the impasse was likely to weigh on the rand currency, briefly boosted last week when there was speculation Mugabe would stand down after his ZANU-PF party lost the parliamentary poll.
"Counting against the rand is the way in which the Zimbabwe elections are rapidly deteriorating into a farce," market analysts ETM said in a trading note.
South African ruling party leader Jacob Zuma on Tuesday criticised the election delay and said it was wrong to keep Zimbabweans and the international community in suspense. He made the remarks one day after meeting Tsvangirai in Johannesburg.
Tsvangirai has appealed for help from outside powers to end Mugabe's uninterrupted rule since independence in 1980.
