Tunisia's main labor union, the Tunisian General Labor Union (UGTT), is holding a vast public sector strike Thursday to demand salary hikes and concessions on looming reforms, upping pressure on Tunisian President Kais Saied.

Up to three million workers could observe the strike, shuttering 159 state agencies and public companies, causing cancellations at airports and bringing public transport, the postal service, and utility firms to a halt.

In a statement on Wednesday, the UGTT union said workers would "hold this strike to defend their economic and social rights after the dithering of the government in the face of their legitimate demands".

Employment Minister Nasreddine Nsibi said the government reserved the right to "requisition" some workers to allow essential services to operate.

While the union insists the strike is not political, it comes as President Kais Saied faces intense criticism for excluding opposition forces from his "national dialogue", part of a push to overhaul the Tunisian state.

The president sacked the government and suspended an elected parliament in July last year, before dissolving the legislature in March and dismissing 57 judges by decree early this month.

The UGTT was invited to take part in the national dialogue, but refused on the grounds that key political forces were not. It also argued that the process aimed to push through "conclusions decided unilaterally in advance".

The union had originally supported Saied’s decision to dissolve the government and suspend parliament, but has become increasingly critical as the president’s measures further expanded to other official institutions.

Tunisians are facing soaring inflation and the UGTT has demanded a new deal to boost public salaries, including retroactively for last year, despite deep public debt.

While its opponents say the union is ignoring the country's deep financial woes, the UGTT is leveraging the government’s need for its support to access the IMF’s bailout.

The government has presented a reform plan to the global lender which includes a freeze on the public sector wage bill, progressive cuts to some subsidies and restructuring of publicly owned companies.

But the UGTT, which has warned against "painful reforms" aimed at pleasing the IMF, has demanded guarantees that state-owned public companies will be spared privatization.

In late May, ratings agency Fitch said tensions between the union and the government were threatening negotiations and warned it would be hard to push through reforms without UGTT backing.

The Labor Union has become a very active and crucial part in the country's political life since its founding in 1946.