February 25, 2017 0350 GMT
Tunisia has witnessed a wave of terrorist attacks since its 2011 revolution, including on foreign tourists, and the United Nations estimates that there are more than 5,000 Tunisians fighting for terror outfits like Daesh, mainly in Iraq and Syria.
"The return of terrorists from hotbeds of unrest in Tunisia is worrying and could lead to the Somalization of the country," said a statement from the internal security forces' national union, AFP reported.
Battle-hardened terrorists "have received military training and have learnt to use all sorts of sophisticated weapons," it added.
The warning came a day after Tunisian authorities said they had arrested three people including the nephew of Anis Amri, a national suspected of carrying out a deadly truck attack on a Berlin Christmas market this week.
Hundreds of people gathered outside parliament in Tunis on Saturday to protest against letting terrorists back in the country.
President Beji Caid Essebsi said earlier this month that Tunisia would refuse to pardon Tunisians who fight for terrorist organizations.
"Many of them want to return, and we can't prevent a Tunisian from returning to his country," he said, "but we will be vigilant."
Following a storm of criticism in the press and on social media, on December 15 he told Tunisian local television that "we will not be indulgent with the terrorists".
On Friday evening, the country's Interior Minister Hedi Majdoub told parliament 800 Tunisian nationals who had fought for terror groups abroad had since returned to the country.