Bennett had initially been charged with treason on Friday, just hours after a unity government of the MDC and Robert Mugabe's Zanu-PF was sworn in.
But the treason charge was dropped on Sunday and replaced with accusations that Bennett allegedly plotted to buy weapons to attack a telecommunications station in Bromley, east of Harare, the capital.
Bennett denies the charges against him.
Bennett had been nominated for the post of deputy minister of agriculture in the new unity government but was taken into custody before any of the cabinet ministers had been sworn in.
The MDC has called Bennett's arrest politically motivated but has so far not acted against the detention.
Tendai Biti, the MDC's secretary-general and newly appointed finance minister, suggested that the party would act if Bennett is not released by Tuesday.
"The prime minister [and MDC leader, Morgan Tsvangirai] is in touch with Mugabe," he said on Monday.
"It is a very, very unsavoury situation ... I hope he will be out by tomorrow [Tuesday]."
Zimbabwe's new government began work on Monday as the MDC considered its response to Bennett's arrest.
Haru Mutasa, Al Jazeera's correspondent in Mutare, said: "MDC supporters and officials are outraged. They want a decision as soon as possible.
"The fact that the new prime minister Morgan Tsvangirai hasn't been able to do this is making people wonder how much power ... he really has in this unity government."
Mutasa said that the issue is expected to be addressed at a cabinet meeting on Tuesday.
Return from exile
Bennett had been living in exile in South Africa for two years after fleeing Zimbabwe to escape charges of plotting to kill Mugabe, but returned last month.
The unity government was formed after nearly a year of political unrest, following disputed elections.
Zimbabwe has come under ever worsening economic and health crises amid the political divisions.
More than 3,400 people have died in a cholera epidemic, while hyperinflation has rendered the Zimbabwean dollar practically worthless.