Tunisia's Prime Minister Beji Caid Essebsi on Wednesday urged parties which pulled out of a reform panel to return and work together in the national interest.
"The national interest requires that all parties work together. One should not quit politics even if there is disagreement," he said at the headquarters of the commission.
Last month, Tunisia's Islamist movement Ennahda (Renaissance) and Moncef Marzouki's Congress for the Republic withdrew from the commission, questioning its legitimacy and accusing it of abusing its prerogatives.
The reform panel was set up in February after the popular uprising that ousted president Zine el-Abidine Ben Ali on January 14, with a brief to pave the way for democratic change in the north African country ahead of the election of a constituent assembly in October.
Essebsi conceded that his interim administration was confronted with security, social, economic and media challenges, but added: "We must be united to deal with this situation."
"There were tensions and disputes with the commission, but those who withdrew must return in the interest of the country," he said.
The reform panel, which comprise representatives of 12 parties, trade unions and associations, is headed by Yadh Ben Achour, a jurist who played a major part in choosing its members, and has set far-reaching goals for the body.
The rules that the commission wants to establish are contested by the large political parties that have substantial resources and do not want to see them cut, according to observers.
There is also a dispute over the inclusion in a "Republican Pact", which is intended to be the basis for a new Tunisian constitution, of a clause banning any normalisation of relations with Israel.