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Tens of thousands took to the streets of towns and cities across France on Saturday in a defiant response to three days of terrorist atrocities that left 17 people dead.

Almost 25,000 attended a march in the southern resort city of Nice, local authorities said, and similar numbers reportedly attended events in Pau and Orleans.

In Paris, mourners left flowers at the scenes of this week’s attacks as they struggled to understand the fear that had gripped the city.

"I don't know what to think," said Asseta Toure, 20, who came to see the makeshift memorial site at Place de la Republique. “I am shocked that there can be terrorists here. It’s the first attack I've lived through."

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She added: "I think behind these three gunmen are hiding the big guys, the more powerful ones who also have arms … who directed the three first men and could have more. I don't think it's finished. It could well be the start of a war."

A two-mile march in Paris from Crowne Plaza Paris-Republique to Place de la Nation is planned for Sunday, and French Interior Minister Bernard Cazeneuve on Saturday night outlined safety precautions that would be in place during the gathering.

Some 2,000 policemen, including 150 plainclothes officers, and 1,350 army personnel would be present. Roof snipers would also be lining the route, Cazeneuve said. He called the terrorism of the last three days “an attack on human rights.”

Matthieu C., 31, lives just down the street from Charlie Hebdo. "We need to continue to live,” he told NBC News. “But we still need to rally and show that we will resist barbarism and fear... And show that we are together in fighting religious fanaticism."

He added that he was impressed by the security forces' response.

“It took three days to find three very isolated guys in a massive region, very densely populated … It gives us confidence in the French police.”

Outside the kosher supermarket in Port de Vincennes were four hostages were killed Friday, police with automatic weapons stood guard as bystanders left flowers.

"It's no longer like before," Maria Pinto told Reuters. "You work a whole life through and because of these madmen, you leave your house to go shopping, go to work, and you don't know if you'll come home."

- Cassandra Vinograd, Emmanuelle Saliba, Alastair Jamieson and Chapman Bell