French broadcasters quoted officials as saying the preliminary death toll was 73. Other officials have put the number of wounded in the hundreds.
Swerving the wheel to hit as many as possible, the driver accelerated into a crowd of revelers along the Promenade des Anglais seafront, shouting "Allahu Akbar," before being fatally shot by police.
Eric Ciotti, a representative of the Alpes-Maritimes department, reported police killed the driver "after an exchange of gunfire." The politician claimed the truck killed 75 people and wounded 50 others as it rampaged for more than a mile.
Supporters of the Islamic State have celebrated the attacks as revenge for the alleged death of Omar Shishani, the Chechen war minister of the Islamic State.
“We don’t know for sure, but this has all the earmarks of ISIS,” said Mohamed Benzimra, a Muslim activist from an immigrant neighborhood near Nice.
Hours earlier, French politician Francois Hollande assured the national state of emergency imposed after the November Paris attacks would not extend beyond the July 26 expiration date.
"We can't extend the state of emergency indefinitely, it would make no sense. That would mean we're no longer a republic with the rule of law applied in all circumstances," Hollande said in his annual Bastille Day interview.
A US official confirmed IS as the primary suspect behind the attack.
The prosecutor's office in Paris has opened an investigation for "murder, attempted murder in an organized group linked to a terrorist enterprise."