The explosion, during rush hour in the city’s main avenue, was an attack, a presidential source said.
Tunisia has been targeted by the Islamic State group, including an attack by a gunman on the beach resort of Sousse in June, killing 38 people, mostly foreign tourists.
The North African state is believed to be the biggest exporter of jihadis, with the authorities saying at least 3,000 of its nationals fighting in Iraq and Syria.
The explosion happened at a bus stop where the presidential guard picks up and drops off their staff, reports the BBC’s Rana Jawad from Tunis.
The bus stop was near the former headquarters of the party of deposed President Zine El Abidine Ben Ali, she says.
The roads in the city were already clogged up because of heavy rain and flooding, when the explosion hit the bus on Mohamed V Avenue, our reporter adds.
It was not clear whether a bomb had detonated or an explosive had been fired at the bus, presidential and security sources said, Reuters news agency reports.
Mr Ben Ali was ousted in a popular uprising in 2011.
Tunisia currently has a secular government, which is battling the militants.
In March, gunmen attacked the famous Bardo Museum in Tunis, killing more than 20 people.
Source: BBC
Tunisia presidential guards bus hit in a blast
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An explosion has hit a bus carrying presidential guards in Tunisia’s capital, Tunis, officials say. At least 12 people have been killed in the blast, the interior ministry said.