A sea of people thronged the Nice promenade for the emotional minute's silence just days after a Tunisian attacker drove a truck into a crowd at the same place on Bastille Day, killing 84 people and injuring around 300.
Similar gatherings were held across the country, with the minute's silence accompanied by the ringing of church bells.
Prime Minister Manual Valls was booed and faced shouts of "resign" as he arrived and left the seaside promenade in Nice, in a sign of the anger and bitterness gripping France after its third major terror attack in 18 months.
He defended government efforts to halt terror attacks, calling for "dignity and truth" from fiercely critical opposition politicians as the national mood soured further nine months ahead of a presidential election.
France was wrapping up a three-day period of national mourning after Lahouaiej-Bouhlel zigzagged a 19-tonne truck through a crowd of tourists, locals and families enjoying a fireworks display in the Riviera city of Nice on Bastille Day.
The attack came eight months after IS jihadists killed 130 people across Paris, and 18 months after three days of terror at the Charlie Hebdo weekly and a Jewish supermarket killed 17.
Former president and main opposition leader Nicolas Sarkozy said yesterday that "everything that should have been done the past 18 months was not done".
"We are at war, outright war. So I will use strong words: it will be us or them," he said.
Cazeneuve described the bitter debate as "shameful".
"Certain members of the political class have not respected the mourning period," he said.
The frustration of the French was writ large in some of the messages left among flowers and tributes on Nice's seafront.
"Enough with the speeches" and "Sick of carnage in our streets", the messages read.
