While it is still not clear what’s happening, you can see from a video taken that there are people laying on the ground, possible shot.

Just two hours ago, the US embassy in Tunis tweeted this about how "Jews, Muslims and Christians have lived side-by side for centuries" 

According to reports from the Arab networks, a number of armed terrorists tried to storm a Jewish synagogue where the event was being held.
There were exchanges of fire between the terrorists and the Tunisian security forces.

No casualties have been reported so far.