He said: "During the investigation, [police] found images of the sites of some foreign embassies. They also confiscated documents containing a few names of foreign diplomats living in Tunisia, and a quantity of explosives."
These recent incidents in Tunisia are unusual in a country generally known as a sleepy holiday destination.
Regional experts say much of the impetus is coming from the Algerian Salafist Group for Preaching and Combat (GSPC), which is allied to al-Qaeda and which called in a web video posted on Monday for attacks against the French and their allies in Algeria.
Security officials suspect the GSPC is attracting growing numbers of Islamist fighters and providing them with military training, noting that there has been a steady flow of North African volunteers to fight US-led forces in Iraq.
Kacem said that the government had been watching the infiltrators since they crossed the Algerian border, and waited for them to gather other members before striking with the help of the army.
He said the group were all Tunisian apart from one Mauritanian.
Two members of the security forces were killed in the clashes and three others injured, he said.