Parisians banded together Sunday in spontaneous celebrations of life in defiance of the attacks — but then panic over firecrackers sent the crowds fleeing, hiding under benches, overturning chairs and bicycles.

Emotions were raw as the French capital entered three days of mourning for 129 people killed for their way of life.

The famed bells of Notre Dame cathedral pealed for 15 minutes in honor of the victims. Police sirens punctuated the melody.

At cafes targeted by extremist gunmen, and at the Republique Plaza in one of Paris' most vibrant neighborhoods, hundreds of people streamed in gradually throughout the afternoon to a makeshift tribute.

A huge banner draped at the bottom of the statue on the plaza reads: "Can't Scare Us."

Then suddenly a noise crackled, apparently firecrackers. Police officers arrived, guns pointed, to investigate.

Everyone ran in every which direction, crying and escaping by adjacent streets, emptying the huge plaza within minutes.

Shouts rang out — "Run!" ''Get out!" ''Lie down!"

People tripped over flowers, candles and souvenirs left in tribute. They took refuge in a nearby hotel, a sports store, under café tables, park benches, behind trees.

"Whoever starts running starts everyone else running," Alice Carton, a municipal worker who came to Republique with two friends, said in a series of messages tapped out over her smartphone. "It's a very weird atmosphere. (The) sirens and screaming are a source of fear."

At a special Notre Dame service for the victims, several lines stretched out of the cathedral. Scores of police patrolled, their hands on their weapons. Police cars surrounded all sides of the 850-year-old monument.

"I came on vacation for five days, I was sorry all this happened but I felt like staying here was supporting France," said Marina Presnyakova, a 37-year-old tourism industry worker from Sochi in Russia.

Michael Staubes, a 70-year-old retired Virginia native now living in Paris, put his hand to his forehead in distress and emotion as the cathedral's bells rang.

"I'm taking a video of the bells in solidarity to show my friends back home in the States. I found the whole situation so disturbing. It's too difficult, there are no words."

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Raphael Satter and Angela Charlton contributed to this report.