About 50 people went on trial for the killing of tourists during an attack on Tunisia's National Bardo Museum in 2015. About 30 are being tried in absentia.
A trial opened in Tunisia on Tuesday over the 2015 attack at the Bardo museum that killed 21 foreign tourists and a police officer, court officials said.
Two gunmen opened fire at the National Bardo Museum in the Tunisian capital city of Tunis in an attack claimed by Daesh.
Some 21 detained suspects, including two women, attended Tuesday morning's unannounced hearing, defence lawyer Samir Ben Amor said.
Three others, who were not under arrest, were not present, he said.
About 30 people are also on trial in absentia, defence lawyer Rafik Ghak said.
The suspects, who were not named, would be tried for "terrorist crimes", according to the French Association for Victims of Terrorism and Imed Belkhamsa, a lawyer for victims of the attacks.
Death sentence possible
Under a 2015 anti-terror law, they could face the death sentence -- although Tunisia has had a moratorium on implementing capital punishment since 1991.
Since its revolution in 2011, the country has faced a series of terror attacks that have claimed the lives of more than 100 soldiers and police, 20 civilians and 59 foreign tourists, according to an official tally.
A month after the Bardo attack, 38 foreign holidaymakers including 30 Britons were killed in a gun and grenade attack on a beach resort near the city of Sousse.
That November, a suicide bombing in the capital killed 12 members of the presidential guard.
Daesh claimed all three attacks.
Following the Bardo attack, authorities arrested around 20 people and announced they had dismantled "around 80 percent of the cell" responsible.
Months later, they released eight of the suspects, including a man they had said was the head of the cell.
French lawyers for the victims and their families have said the investigation had left "several dark areas".
One of them, Philippe de Veulle, said he would boycott the trial, saying it would not offer "independent justice".
Some 26 people went on trial in May over the Sousse attack, including six security personnel accused of failing to assist people in danger.
More than 5,000 Tunisians have travelled abroad to join militant groups, mainly in Iraq, Syria and Libya.