French police officials' murder 'is terrorism' - Hollande
A police commander was stabbed to death before his partner - also a police employee - and a child were taken hostage in their home near Paris.
The partner was later found dead but the three-year-old boy survived.
The attacker had spent time in jail over links to jihadist groups.
He was killed during the police assault on the couple's home in Magnanville.
Named by French sources as 25-year-old Larossi Abballa, he lived in nearby Mantes-La-Jolie and was convicted in 2013 of "criminal association with the aim of preparing terrorist acts".
Abballa was sentenced to a three-year term in prison, of which six months were suspended, for recruiting fighters for jihad in Pakistan.
The IS-linked Amaq news agency said an Islamic State "fighter" had carried out the attack.
France has been under a state of emergency since the Paris attacks of 13 November 2015 in which 130 people died.
That state of emergency is still in place with France on high alert as it hosts the Euro 2016 football championship, which started last Friday.
President Hollande said the 42-year-old policeman, named as Jean-Baptiste S, and his 36-year-old partner, Jessica S, were "murdered in cowardly fashion."
"It's unquestionably a terrorist act," Mr Hollande said, stressing that France was still "facing a very significant terrorist threat".
Earlier, Interior Minister Bernard Cazeneuve said more than 100 people seen as potential threats had been arrested in France this year, including in recent weeks.
'Loud explosions'
French prosecutors have launched an anti-terror investigation into the incident.
Witnesses said that the knife-wielding man may have shouted "Allahu akbar" (God is great) when he attacked the policeman, who was not in uniform, outside his home.
The attacker then went inside the house and took his partner and child hostage.
Interior ministry spokesman Pierre-Henry Brandet said France's elite police Raid unit was brought in after the officer's killer took the hostages at around 21:00 (19:00 GMT).
Abballa was said to have posted a long video from the scene as well as pictures of his victims on his Facebook page, which was suspended on Tuesday morning.
Mr Brandet said negotiations were unsuccessful and a decision had been made to "launch an assault" at about midnight.
Witnesses reported hearing loud explosions as the officers from the Raid unit moved in.
"This commander, this police officer was killed by the individual... (and) we discovered the body of a woman. The assailant, the criminal was killed. Thankfully, a little boy was saved. He is safe and sound," Mr Brandet told reporters.
Magnanville is about 55 km (35 miles) north-west of the French capital.
Source: bbc.com