Tunisians are preparing to vote in the first elections since President Zine al-Abidine Ben Ali was ousted as part of Arab Spring protests.
There are reports of long queues in polling booths around the country on Sunday.
Prime minister David Cameron said it was "inspiring" that candidates were coming forward and that Britain would support them as they moved towards a "free, democratic and inclusive future".
"For the first time today, Tunisians are able to express their views at the polling booth in what I hope will be free and fair elections. It is inspiring that so many candidates are competing for the opportunity to draw up the country's new constitution. This remarkable achievement reflects how far the Tunisian people have come since January when they rose up against their autocratic ruler to demand their right to freedom, dignity and a better future.
"Their determination and courage inspired real change across the Middle East and North Africa. As the first country in the region to put democracy to the test at the polling booth, Tunisia is once again leading the way.
"While Tunisians cast their votes today, Libyans are celebrating the end of forty years of ruthless dictatorship. Both have shown what can be achieved and just as we have stood by them in their fight for freedom so we will continue to support them in shaping a free, democratic and inclusive future."
An Islamist party, Ennahda, is expected to win the vote. Their leader Rachid Ghannouchi said it was a "historic day" as he queued to vote. “Tunisia was born today. The Arab Spring was born today.” However there are questions over his party. Reuters reported:
Islamist leader Ghannouchi, who spent 22 years in exile in Britain, has associated his party with the moderate Islamism of Turkish Prime Minister Tayyip Erdogan. He has said he will not try to impose Muslim values on society.
But the party’s rise is worrying Tunisia’s secularists who believe their country’s liberal, modernist traditions are now under threat.