Last year's election run-off was marred by state-sponsored violence
Zimbabwe's Movement for Democratic Change has said there are "increased violent" attacks on its party members.
MDC spokesman Nelson Chamisa said a top official was stopped and beaten up by militants from President Robert Mugabe's Zanu-PF on Tuesday morning.
His warning comes a few days after an MDC residence was raided by police.
Zanu-PF has described the comments as "cheap propaganda" following the MDC's decision to end co-operation in the unity government formed in February.
Mr Chamisa told journalists in the capital, Harare, that acts of violence against MDC supporters had started happening in both urban and rural areas.
The pattern of violence was reminiscent of the attacks on MDC supporters during last year's elections, he said.
On Tuesday, Prime Minister Morgan Tsvangirai, the MDC leader, boycotted a cabinet meeting for a second time in as many weeks in protest at Zanu-PF's perceived failure to implement measures agreed to as a part of the power-sharing deal.
The MDC has threatened to call for fresh elections if a meeting of the Southern African Development Community (Sadc) later this week fails to break the deadlock.
Sheila Mashaire, a senior MDC official with the party's security department, said was stopped on her way to party headquarters early on Tuesday and beaten by armed men who said they wanted to arrest her.
Mr Chamisa said the men were Zanu-PF militants, who fled the scene when a crowd gathered.
He said this could be the beginning of worse things to come, and the depth of the crisis could not be underestimated.
"We take this very seriously. We are possibly on the brink of another storm of persecution and intimidation," Mr Chamisa noted.
Although Zanu-PF has not commented officially, one senior member - Ephraim Masawi - denied the MDC allegations, describing them as "cheap propaganda".
It is thought that Zanu-PF may issue a statement later on Tuesday.
But political analysts in the state-controlled Herald newspaper say the decision by the MDC to disengage from government was unfortunate, and borders on the party's inability to negotiate.
Local journalist Brian Hungwe told the BBC that ordinary Zimbabweans are anxiously waiting for a resolution to the crisis as hopes have been raised and dashed countless times before.