PARIS — French police have arrested a 15-year-old boy at his Paris home to thwart what they feared was a planned weekend attack, as the prime minister warned on Sunday some 15,000 people in the country could be in the process of being radicalized.

The arrest of the teenager on Saturday came two days after police moved in on what the Paris prosecutor says was a group of female “commandos” arrested after an aborted attack at Notre Dame Cathedral and another possible attack. Those arrested included a 15-year-old girl, the daughter of one of three women arrested south of Paris.

A security official said Sunday that France’s intelligence services “detected a threat” and a judicial official said police moved into action Saturday fearing a planned attack this weekend.

The officials said the boy’s arrest was not linked to the arrests last week of four women, the 15-year-old daughter of one of the women and a man. So far, one woman has been charged.

Intelligence agents suspected the boy planned to carry out a knife attack in a public place this weekend, the judicial official said, refusing to name a spot where it was thought the attack might occur.

France is in a state of emergency after three attacks this year, including the Bastille Day truck attack in Nice that killed 86 people. That followed two waves of attacks last year, notably the Nov. 13 attacks on restaurants, bars, a concert hall and stadium that left 130 people dead.

News of the arrest came shortly after Prime Minister Manuel Valls said Sunday that “every day attacks are foiled … (including) as we speak.”