Tourists were being warned to avoid large parts of Tunisia as the threat of terrorism in the country heightened.
The warning, issued by the Foreign and Commonwealth Office (FCO), follows attacks by dissidents, who reportedly killed 14 soldiers and injured 20 more in the Chaambi Mountains in the far west of the country, near the Algerian border.
The popular holiday destination attracted more than 400,000 British tourists last year, but travelers were being warned to be "extra vigilant".
An FCO spokesman said: "Our advice makes clear that there is a high threat from terrorism in Tunisia.
"We continue to advise against travel to the Chaambi Mountain National Park area."
Other areas to avoid included the towns of Nefta, Douz, Medenine and Zarzisj, and to within 30km of the border with Algeria from south of the town of Ghardinaou.
Tunisian President Moncef Marzouki and the head of the National Assembly have signed the country's new constitution, officially adopting a charter that is one of the country's last steps to full democracy after a 2011 uprising.
Tens of thousands of Tunisians crowded the streets of Tunis to demand the government's ouster, in the largest opposition protests to hit the capital since the country's political crisis began two weeks ago.
The secular opposition is trying to topple the Islamist-led government and dissolve a transitional Constituent Assembly that is only weeks away from finishing a draft constitution and new election law.
The protest marks the six-month anniversary of the assassination of leftist politician Chokri Belaid, one of two opposition figures to be shot dead in recent months.
Tunisian police fired teargas to disperse violent protests in the southern town of Sidi Bouzid, cradle of the country's revolution and hometown of slain opposition figure Mohamed Brahmi, witnesses told Reuters.
Angry demonstrators, who came out to protest Thursday's assassination of Brahmi, threw rocks at police, said local resident Mahdi al-Horshani.
"Hundreds of protesters lit tyres on fire to block roads and they threw rocks at the police," he said. "There is a lot of anger and frustration at the situation."