Tunisia's powerful general labour union said on Monday that it would hold a national strike over wages and the economy.
The union refused to take part in a limited dialogue proposed by President Kais Saied as he rewrites the country's constitution.
With more than a million members, the UGTT is Tunisia's most powerful political force and its call for a strike may present the biggest challenge yet to Mr Saied after his seizure of broad powers last summer.
He has remained focused on his political agenda, composing a new constitution for what he has described as a "new republic" in Tunisia, since closing Parliament in July.
Mr Saied discounted most of Tunisia's constitution to say he would rule by decree despite a gathering economic crisis.
An online consultation, conducted between January and March this year, was designed to divine the political desires of Tunisians.
It had poor participation, with fewer than 7 per cent of eligible voters taking part. There were also concerns about data security.
Mr Saied has praised its results and said he would use them to inform his new constitution.
He called for a national dialogue to be held that would include the UGTT and three other civil society organisations that formed the Nobel Peace Prize-winning quartet that salvaged the country's 2014 democratic transition.
The union rejected Mr Saied's proposal after he said last week that political parties would be barred from a role in forming the new constitution.
It would replace the 2014 document that emerged from an inclusive debate among Tunisia's main political factions and social organisations.
"We reject any formal dialogue in which roles are determined unilaterally and from which civil and political forces are excluded," UGTT spokesman Sami Tahri said.
The forceful rejection and call for strikes by by UGTT members in public services and state companies is a departure from the union's earlier hesitancy to take a stand against Mr Saied.
In the months since he assumed sole control, it has cautiously tried to speak with the president and refrained from outright condemnation.
Achaab, the newspaper of the union, said that Mr Saied met UGTT leader on Sunday and told him that he insisted the dialogue would be in the current formula that he proposed.
The date of the strike will be announced later, Mr Tahri said.
Strikes called by the UGTT have throughout the course of Tunisia's history been turning points in political and social movements.
They include the 2011 uprising, when a general strike brought hundreds of thousands to the streets in an attempt to overthrow then president Zine El Abedine Ben Ali.
Mr Saied's government is in talks with the International Monetary Fund for a bailout, regarded as being necessary to ward off national bankruptcy.
But the UGTT rejected proposed spending cuts and wants wage increases for state workers.