Tunisian Prime Minister Mohamed Ghannouchi said on national television that Ben Ali had left the country and that he, Ghannouchi, would temporarily lead the north African country, with support from the army, dpa reported.
It was not clear whether Ben Ali had been forced to leave or had agreed to leave of his own accord.
"A dark and bloody page in Tunisia's history has been turned with the departure of the dictatorship," Maya Jribi, secretary general of the Progressive Democratic Party, the country's main opposition party, told the German Press Agency dpa by telephone.
"We demand to form immediately a coalition government to put Tunisia on the proper path of democracy and freedom."
The news was met internationally with calls for Tunisians to form a new government in as peaceful a manner as possible.
"We will use our influence to make sure that the situation there is settled as peacefully as possible and with as little loss of life as can be," said German Chancellor Angela Merkel Friday night.
Ben Ali, a 74-year-old former interior minister, had been president of the north African country since 1987, replacing self- styled "president for life" Habib Bourguiba when he was deposed in a coup.
Al-Jazeera television network reported that Ben Ali had fled to Paris. It was not clear whether France was his final destination. Some reports suggested he had been offered refuge in Libya, others in Malta, though Maltese government officials denied Ben Ali's presence there Friday evening.
His departure comes after a month of anti-government demonstrations that were brutally repressed by the police.
More than 60 protesters have been killed since mid-December, most shot dead by police firing on crowds with live bullets.
A state of emergency was declared in Tunisia on Friday evening. The army took control of the main airport in Tunis and Tunisian airspace was closed.
Thomas Cook travel agency evacuated 2,000 German tourists from Tunisia. France's BFM television reported that a French plane carrying tourists to the north African country had turned back after Tunisian airspace was closed.
As a last gesture in a day where tens of thousands of people had again demonstrated nationwide, Ben Ali had fired his government and announced early elections.
But the demonstrators demanded nothing less than his own immediate exit.
Friday's protest in Tunis began peacefully but descended into chaos after the police fired tear gas into the crowd, causing a stampede that reportedly killed four people.
BFM television also reported that a policeman had shot a photographer in the head.
Ben Ali, who has been in power for 23 years, had been on the ropes since the authorities began firing on demonstrators protesting unemployment and high food prices since mid-December.
The police brutality shocked citizens of the habitually stable country, which lives mainly off tourism and agriculture, and fanned the flames of protest.
On Thursday Ben Ali was still trying to cling to power until the end of his mandate in 2014, telling Tunisians in a television address he was committed to democracy and "very sad" about the civilian deaths.
Yet, even after his television appearance, police continued to shoot at rioters who defied a curfew in Tunis, killing 13 people in two separate neighbourhoods.
The protests began in the central town of Sidi Bouzid on December 17, with the self-immolation of a desperate, hard-up vegetable vendor outside a government building.
His action ignited a wave of protests that tapped into widespread discontent over high ýouth unemployment, rising food prices and rampant corruption.