Tunisia's powerful labour union attacked the president's political and economic agenda on Saturday, including elections this month, saying it will no longer accept what it called a threat to democracy.
The UGTT union says it has more than a million members and has proven able to paralyse the economy with strikes. It has at times backed President Kais Saied after he seized most powers last year, but on other occasions has voiced tepid opposition.
"We no longer accept the current path because of its ambiguity and individual rule, and the unpleasant surprises it hides for the fate of the country and democracy," UGTT's leader Noureddine Taboubi said in a speech to thousands of supporters.
"We will not hesitate to defend rights and freedoms whatever the cost," he added, in his strongest criticism yet of the president.
Mr Saied shut down the elected parliament last year and moved to rule by decree before writing a new constitution that was passed this summer in a referendum with low turnout, setting up elections for a new, weakened legislature on December 17.
Most political parties are boycotting the poll, saying the new parliament will have no power and faulting procedures the president has decreed, which include bringing the electoral commission under his purview.
Mr Taboubi said the December election would "have no colour and taste" as a result of Mr Saied's constitution and that the vote lacked national unanimity.
The president's critics have denounced his moves as a coup and have held repeated street protests. Mr Saied says his actions were necessary to save Tunisia.
Although the UGTT has previously voiced concern, it has stopped short of openly opposing Mr Saied's agenda, except for a strike in the summer over wages and spending cuts.
This year, as the economy worsened, the new government Mr Saied appointed angered the UGTT by proposing subsidy cuts and the restructuring of state-owned companies in a push for an IMF bailout needed to avert national bankruptcy.
"We will not abide by secret agreements the government has with the International Monetary Fund and the workers will stand up to it," Mr Taboubi said.