Six security officers and 20 other people are on trial in Tunisia over the 2015 Islamic extremist attack on a beach resort that killed 38 people, mostly British tourists.
Tunisia's tourism industry has yet to recover from the massacre, and the country still struggles against extremist violence.
The attacker, a Tunisian student who trained with Libyan militants, was killed by police after the attack on June 26 2015.
Islamic State claimed responsibility for the attack on the Imperial Hotel in the Mediterranean resort of Sousse.
The officers who went on trial in Tunis are accused of failing to prevent or stop the attack.
The other suspects face accusations including involvement in preparing or inciting the violence.
The unusually large trial includes around 70 lawyers as well as British Embassy representatives and foreign observers.