"This attack is fresh proof that we must prepare for other attacks and therefore protect ourselves," he told diplomats gathered in Paris on Tuesday.
Ayoub El Khazzani, 25, boarded the high-speed Thalys train in Brussels on Friday armed with a Kalashnikov assault rifle, a Luger automatic pistol, ammunition and a box-cutter.
Under French law, authorities can question Khazzani for four days after detaining him, a deadline that expires Tuesday evening. At that point they can charge him or apply for an extension.
Khazzani, who authorities say does not speak French, insists he only sought to rob passengers and that he found the weapons stashed in a park in Belgium where he sometimes slept, a lawyer temporarily assigned to his case told media over the weekend.
But Khazzani was on the radar of European intelligence agencies in Spain, France; Belgium and Germany for ties to radical Islamist movements.
A Spanish counter-terrorism source told news agency AFP that Khazzani had lived in Spain for seven years until 2014, where he came to the attention of authorities for making hardline speeches defending jihad and attending a radical mosque, and also for his involvement drug trafficking.
French intelligence sources, meanwhile, have said he showed up on their radar in May this year, when German authorities warned he had boarded a plane for Turkey, seen as a possible sign that he travelled to Syria.