The Fiji Labour Party wants a revised plan for the new constitution.
The Fiji interim government's year-long timetable includes civic education and consultation by a regime-appointed Commission.
A yet-to-be-appointed Constituent Assembly will give final approval.
But Labour Party leader Mahendra Chaudhry says the process is not inclusive or participatory as promised by the regime.
He says his party will get together with other parties, civil society and unions to campaign for a change in the plan.
"The point is that we've not been included in the formulation of the process and therefore we have to have an alternative to counter that. So what we want to do is let the people, when they appear before the commission, articulate their views and that they are properly advised over that."
Mahendra Chaudhry accepts that the constitution process is inevitable.
Some groups including unions and other political parties say it is illegal as Fiji already has the 1997 constitution which was abrogated by the regime in 2009.