Tunisia on Monday installed as its new president a former dissident who was imprisoned and then exiled for opposing former President Zine al-Abidine Ben Ali, a new landmark in the country's post-revolutionary transition to democracy.

Members of the constitutional assembly, Tunisia's interim parliament, voted to elect Moncef Marzouki as president, the second most powerful role after the prime minister.

Marzouki, 66, is respected by many Tunisians for his implacable opposition to the autocratic Ben Ali. As president, he will be a secularist counterweight to the moderate Islamist party which is now Tunisia's dominant political force.

Tunisia became the birth-place of the Arab Spring uprisings in January when protests forced Ben Ali, in power for more than 23 years, to flee to Saudi Arabia.

That inspired revolutions in Egypt and Libya, as well as unrest in other Middle Eastern states.

I promise the Tunisian people that I will work for the country with all my strength, Marzouki said after the vote. I represent a country, a people, a revolution. Long live Tunisia.

I say to those members who gave me their votes, thank your for your trust, and for those who did not vote for me, your message has been received .. I know that you are going to hold me to account, Marzouki said.

Marzouki, who was elected with 153 of the 202 votes cast, will serve for a year until the constitution is re-written and new elections are held.

About 40 opposition members of the assembly cast blank ballots in protest at a vote they said was a charade to mask the fact that real power was now held by the Islamists.

This was a piece of theatre, said Najib Chebbi, head of the PDP party. We are disappointed in Mr Marzouki that he has accepted a presidency which is just democratic window-dressing without any real functions.

Secularist politicians say the Islamists will undermine Tunisia's liberal values and impose a strict moral code. Ennahda denies it has any such intentions, saying instead it will follow the moderate example of the Islamists who rule Turkey.

PRISON TERM

A doctor and human rights campaigner, Marzouki was jailed in 1994 after challenging Ben Ali in a presidential election.

He was released four months later when his case became the focus of an international campaign, but was forced to go into exile in France.

Marzouki returned home three years before the revolution but left again about two months later, saying he could not operate because of harassment by the authorities. At that time, hundreds of plainclothes police officers surrounded his home and office around the clock and followed him to meetings.

Days after protests forced Ben Ali to flee on January 14, Marzouki flew home from Paris and was greeting at Tunis-Carthage airport by cheering and singing followers.

Marzouki was elected president as part of a power-sharing deal between the moderate Islamist Ennahda party and its two smaller secularist coalition partners, Ettakatol and Marzouki's Congress for the Republic.

Ennahda and its two allies won a majority in the constitutional assembly when Tunisia held its first democratic election in October.

Under the coalition deal, Ennahda's secretary-general Hamadi Jbeli will hold the most powerful position, of prime minister, while Ettakatol leader Mustafa Ben Jaafar becomes speaker of the constitutional assembly.

The arrangement gives the president limited powers. He sets Tunisia's foreign policy in consultation with the prime minister. He is also commander-in-chief of the armed forces but can only appoint or fire senior officers in consultation with the prime minister.

It's an historic moment to have a first legitimate president of the country, Samir Dilou, a senior Ennahda official, told Reuters.

No one can deny that this moment has just laid another stepping stone on the path towards real democracy in Tunisia and in the region.

(Writing by Christian Lowe; Editing by Alistair Lyon)