Mohamed Ghannouchi said in an interview on Tunisian television Friday he will leave power after a transition phase leading to legislative and presidential elections "in the shortest possible timeframe."
Protesters have been demanding for days the departure of all remnants of the old guard under ousted President Zine El Abidine Ben Ali. Ghannouchi was an ally of Ben Ali and has been struggling to restore calm under a new multiparty government.
"My role is to bring my country out of this temporary phase and even if I am nominated I will refuse it and leave politics," Ghannouchi said.
Ghannouchi did not say why he is leaving politics or specify when the elections would be held. He said the elections must be a success "to show the world that our country has a civilization."
The prime minister also pledged that all of the assets held abroad by Ben Ali's regime had been frozen and would be returned to Tunisia after an investigation. He did not elaborate.
Tunisians began three days of mourning Friday, lowering flags and broadcasting recitations of the Quran to mourn dozens who died in the protests that drove their autocratic leader from power.
Earlier in the day, about 1,000 demonstrators gathered in the courtyard of Ghannouchi's offices, some climbing lampposts and hanging Tunisian flags and hand-lettered signs.
Central Tunis has seen near-daily protests in the past week by those who say the caretaker government is still too dominated by cronies of ousted President Zine El Abidine Ben Ali.
The government has declared three days of national mourning as it struggles to restore calm and reconcile this Muslim nation in North Africa, whose people are hopeful yet scarred by 23 years of limited freedom that ended when Ben Ali fled to Saudi Arabia.
Tunisia is also a beach and desert haven for European tourists and U.S. ally in the fight against terror.
Friday's protest in Tunis, the seaside capital, started at the long-dreaded Interior Ministry then moved to Ghannouchi's offices. The wall of one building was covered with anti-government and pro-revolutionary graffiti, including "Death to the Dictatorship" spray-painted in black letters.
Police in some locations were joining ranks with protesters. Officers climbed atop their patrol cars with protesters, waving flags and chanting along with demonstrators outside the prime minister's office. One policeman was lifted onto the shoulders of demonstrators and was seen chanting pro-police slogans along with the crowd.
Outside the Interior Ministry, a policeman in tears took off his uniform cap and joined the marching demonstrators to applause by protesters.
Interior Minister Ahmed Friaa asked Tunisians to be patient with the new government, which was installed Monday.
"Just wait a few days we have waited years, just wait a few days," Friaa said.
The government says 78 civilians were killed, many shot by police, in nearly a month of protests over unemployment, corruption and repression that eventually forced Ben Ali to flee. A French photographer was also killed as were some security forces, and scores of people were injured. Opposition members say the death toll was much higher.
On Friday, flags were lowered at public buildings around Tunisia, and television showed little but Quran recitations. In Tunis, however, many shops and businesses opened as usual.
The protest near the Interior Ministry was almost festive, with women ululating and crowds carrying balloons. Omar Shahbani, a 43-year-old auditor, carried 15 balloons of different sizes and colors.
"I brought the beautiful colors of the Tunisian people," he said. "The balloons make people happy and remind us of the festival that freedom is."
He said the mood was similar to 1987, when Ben Ali took power in a bloodless coup.
"I was 23 in 1987 and today I feel like I'm 23 again. My youth has returned to me with this revolution," he said.
Some protesters held a sign in English referring to President Barack Obama's "Yes, we can" campaign slogan: "They said 'We can,' Tunisians say, 'We DO!"
The new central bank president denied that Tunisian government's stock of gold was taken during the final days of Ben Ali's reign.
"There has been no change in our stock of gold and its weight has been the same for years," Mustapha Kamel Nabli said at a news conference.
Nabli said Tunisia holds 5.3 tons of gold in its own vaults and another 1.397 tons placed with the Bank of England.
Nabli replaced Taoufik Baccar, who resigned as central bank head this week following rumors that the ex-president's wife fled with a huge stash of gold.