Five secular Tunisian parties merged on Saturday in an attempt to create a rival to Ennahda, the Islamist party that swept to power in the first free elections last year.

Ennahda, which was banned under ousted president Zine al-Abidine Ben Ali, won more than 40 percent of seats in a new constituent assembly last October and has gone on to form a government in coalition with two non-religious parties.

While a large number of secular parties split the liberal and leftist vote, Ennahda, as the only major religious party standing, scooped much of the conservative vote.

"We have a historic opportunity to shift the balance of power and to fight a battle that will decide the future of Tunisia in the medium and long term," said Yousef al-Shahed, head of the Republican Party, a constituent of as-yet-unnamed new party.

Even banded together, the new party does not come close to challenging Ennahda in terms of seats in the constituent assembly.

The new parliament will replace the assembly, whose main task has been to draft a new constitution in the wake of Ben Ali's overthrow and subsequent exile in Saudi Arabia.

The new alliance brings together a number of prominent secular groups, chief among them the Progressive Democratic Party (PDP) and Afek Tunis, which together won just under 10 percent of assembly seats.

"This initiative to combine the modernist centrist parties aims to create a balance of power and to prepare us better for future elections," PDP leader Nejib Chebbi told Reuters at a rally launching the party. "But bringing together the largest number of forces is not enough. We must also develop our message and reach all deprived groups."

Several former ministers and other senior politicians from Tunisia's interim administration have joined the new party and talks are taking place to bring in the secular leftist Ettajdid, or Renewal, party.