"After the transition, I will retire from political life," said Ghannouchi in comments broadcast on Friday. He has been Prime Minister since 1999.
"All undemocratic laws will be scrapped" during the transition to democracy, he added, referencing electoral, anti-terrorism and media laws.
He promised to organise the country's first fair polls since independence from France in 1956 without giving a specific date for them. He previously had said there would be an election within six months; the constitution says it should take place within two months.
The UGTT, the country’s main trade union, has rejected the government installed after 23-year president Zine El Abidine Ben Ali left his post on 14 January because some of the same people still hold key posts.