I was at a peaceful Paris protest. Then the violent far-left mob arrived.

PARIS — I was standing in the middle of the street in east Paris, surrounded by mostly masked protesters, when my throat started suddenly burning.Out of seemingly nowhere, I was coughing uncontrollably and felt my throat closing up.

That was the first tear gas fired of the night.I hurriedly reached for the gas mask I’d carried in my backpack the whole day throughout what up to that point had been a large but peaceful march. Saturday’s protest, the main one in Paris, started at the Porte d’Italie, a largely residential neighborhood on the left bank of the Seine.

Police changed the approved route at the eleventh hour Friday night, saying the original one, meant to go from Place de La Bastille through Place de la Nation in central Paris, would be too difficult to secure.And indeed, along this new route, there were barely any shops for the protesters to loot, as they had famously done the weekend before.Instead were mostly working-class high-rise apartment buildings, where residents, not used to seeing political action in this part of town, came out on their balconies, some with a cigarette in hand, to peek at the tens of thousands marching through.By the time it reached its end point, after nearly three hours, at Porte de Vincennes, riot and military police were making clear that – unlike the previous weekend where the police response was criticized – they were the ones calling the shots this time, advancing on the crowd whenever it looked like it would get out of control.And things remained orderly for nearly another hour – people were chanting, some were jumping and dancing – and we wondered whether we should move to see what was happening in other parts of the city.But then, as many of these peaceful protesters were hopping on the metro to head on home, others were just arriving.

And this late crowd looked different.Their faces were covered, gas masks were already on, they weren’t carrying any signs showing which cause they were fighting for ...