He earlier said a negotiated settlement was the way forward, but violence must stop first.
But Mr Chamisa said the ruling Zanu-PF's militias should be disbanded, adding that his party had received no direct approaches from mediators since the announcement.
The move has sparked international criticism of Zimbabwe's government.
But President Robert Mugabe has blamed the MDC for the recent violence.
The leader of neighbouring Zambia, Levy Mwanawasa - who is head of the regional Southern Africa Development Community (Sadc) - said a vote held in current conditions would be an "embarrassment" to the region.
Meanwhile, African Union Commission chairman Jean Ping said he was worried about the situation.
"This development and the increasing acts of violence in the run-up to the second round of the presidential election are a matter of grave concern to the Commission of the AU," he said.
FROM THE TODAY PROGRAMME
The US and UK have said they are prepared to bring Zimbabwe before the UN Security Council over the election violence.
But South African President Thabo Mbeki, who is mediating in the crisis, called for further dialogue.
"From our point of view it is still necessary that the political leadership of Zimbabwe should get together and find a solution to the challenges that face Zimbabwe," he said.
Zimbabwe's electoral commission said that preparations for the election were under way and a credible result was still possible.
Mr Chinamasa said Zanu-PF was not treating Mr Tsvangirai's "threats" to withdraw seriously.
KEY POLL COMPLAINTS
Violence: 86 killed, 200,000 displaced
MDC rallies banned, leaders detained
Opposition areas not given food aid
State media refused MDC adverts
Zanu-PF supporters to be used as election officials
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