UPDATE 5 p.m. EST: The death toll from an explosion on a bus carrying members of Tunisia's presidential guards in Tunis, the capital, has risen to at least 12, the New York Times reported late Tuesday afternoon. In response to the attack, President Beji Caid Essebsi declared a 30-day state of emergency and announced a nightly curfew beginning at 9 p.m. 

Original story:

At least 11 people were killed in the Tunisian capital Tuesday after an explosion hit a bus carrying Tunisian presidential guards, Reuters reported. A presidential source told Reuters the explosion was "likely caused by a bomber detonating explosives inside the vehicle" as it traveled along Mohamed V Avenue, a major street that runs through the downtown area. 

Security forces sealed off the area and medical experts quickly responded to the scene. Hundreds of police and soldiers swarmed the area, Al Jazeera English reported

"It's in the heart of downtown -- a residential area -- and many people who live around here are gathering," a local journalist, Aymen Abderrahman told Aljazeera. "People are crying because it's the first time it's happened in downtown ever.”

If confirmed as a terrorist attack, it would be Tunisia’s third major attack this year. Two previous major attacks targeted the country’s tourism industry. In one of those incidents, a gunman killed 38 tourists at a beach resort in June, and earlier in the year, gunmen targeted tourists at a museum, killing 21 people. The Islamic State group claimed responsibility following both attacks.

Tunisia has seen sporadic political violence in recent years. It has been hailed as a success of the Arab Spring – the wave of protests that broke out across much of the Middle East in 2010 and 2011 – after protesters took to the street to demand the ousting of the longtime president. The protests succeeded in ushering in a new democratic government, but they didn't succeed in bringing calm. The army has been battling Islamic fighters in the mountains near the Algerian border, and militants have in the past targeted military posts in rural areas.  

“Tunis” was trending on Twitter Tuesday, as people across the world took to the social media platform to voice their support for the Tunisian people with the hashtag #PrayForTunis.