Tunisia has declared a state of emergency, just over a week after 38 tourists, mainly Britons, died in an attack in the resort city of Sousse.
The state of emergency gives security forces more powers and limits the right of public assembly.
Authorities had already tightened security in the wake of the attack on 26 June, deploying more than 1,400 armed officers at hotels and beaches.
Tunisian President Beji Caid Essebsi is due to address the nation later. Security forces were criticised for not responding more quickly to the attack in Sousse, when a gunman opened fire on tourists on a beach and in a hotel before being shot dead by police.
The gunman has been identified as student Seifeddine Rezgui, who authorities say had trained in Libya.
Tunisian Prime Minister Habib Essid conceded in a BBC interview on Friday that the slow response of the police was a key problem.
He said Rezgui had probably trained with the Ansar al-Sharia group, though Islamic State (IS) earlier said it was behind the attack.