The heads of 21 political parties were to gather Monday to try to agree on a new prime minister to be named this week to form a cabinet of independents.
Following a months-long political crisis, the government led by the moderate Islamist movement Ennahda is to be replaced under a series of talks launched on Friday.
The meeting of party leaders was to start at 1500 GMT, according to the UGTT trade union which has been the main mediator of the "national dialogue" between the government and opposition.
Under the timetable of the talks, the new prime minister -- a post for which no candidate has been declared -- will have two weeks to form a government.
The elected Constituent National Assembly has until the end of November to draw up a new constitution, an electoral law and to form a commission to organise polls.
Incumbent Prime Minister Ali Larayedh has pledged to step down as long as the timetable is respected.
The murder in July of an opposition leader, Mohammed Brahmi, by suspected jihadists triggered mounting calls for the resignation of his Ennahda-led government.
Since the 2011 uprising that ousted Zine El Abidine Ben Ali's regime, the country has been rocked by violence blamed on jihadist groups suppressed under the veteran strongman.