At least 12 people have been killed in an explosion targeting a bus carrying presidential guards in the Tunisian capital, the interior ministry said, calling the incident a "terror act".

The bus was hit on the busy Mohamed 5 Boulevard in central Tunis on Tuesday evening. 

President Beji Caid Essebsi declared a state of emergency after the attack and said the country is "at a state of war". A curfew will be in place in the Tunis region until 5am on Wednesday morning.

Local journalist Aymen Abderrahman said that "hundreds of police and military" swarmed the area of the attack after the explosion. 

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"It's in the heart of downtown - a residential area," he told Al Jazeera by telephone, adding that people gathered and were "crying because it's the first time it's happened in downtown ever".

"The blast was heard throughout the city," Abderrahman said. 

Emir Sfaxi, a Tunis-based activist and blogger, described traffic jams and a heavy military presence in the city after the attack.

"There is a lot of confusion," he told Al Jazeera shortly after the incident, adding that it took place "near the Ministry of Interior".

The presidential guard is an elite security force that protects only the president.

Armed groups have targeted Tunisian security forces and civilians in the past, including in the capital and in areas in the country's central region and on the border with Algeria. 

Earlier this week, the interior ministry said that former President Moncef Marzouki was the target of an assassination plot. 

Marzouki "was informed by the interior ministry that he had been targeted in an assassination plan", a ministry spokesman told AFP, adding that it was planned by a "terrorist" group. 

Instability

Since Tunisians toppled the 23-year presidency of strongman Zine El Abidine Ben Ali in 2011, the country has held several democratic elections. 

But it has also endured several attacks on state security forces and civilians, including foreign tourists. 

In June, gunmen killed 38 European tourists at a beach resort hotel in the coastal city of Sousse. The Islamic State of Iraq and the Levant (ISIL) took credit for the attack, while government officials claimed the perpetrator was the local Ansar al-Sharia armed group. 

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That attack came just three months after an attack on the Bardo National Museum in March, which left 22 people - among them tourists - killed. The attack is believed to have been carried out by Okba Ibn Nafaa, a local affiliate of al-Qaeda. 

Tunisia has fared better than many of the Arab countries where revolutions launched in 2011 resulted in military coups, such as in Egypt, or prolonged violence, like in Libya and Syria. 

But Tunisia has seen instability as it struggles with a post-revolutionary economy and years of political turmoil, including political assassinations that killed two major opposition figures, popular left-wing politician Shokri Belaid and progressive leader Mohamed Brahimi.

Source: Al Jazeera