The sound of a police siren doesn't usually turn heads in a big city like this, but this is no normal weekend in the French capital.

It's when people stop and look up with a worried expression on their faces whenever a police car races past that you feel the tension in a city reeling from three terrorist attacks in as many days; at a satirical magazine, on a policewoman responding - she thought - to a regular traffic incident, and at a kosher supermarket in the east of the city.

The site of the first attack at the offices of Charlie Hebdo, where 12 people were gunned down by the Kouachi brothers, is still sealed off.

At the roadblocks in the side streets around the crime scene flowers, drawings and messages to the victims have been placed.

Like the one from nine-year-old Jules - a child's drawing of a happy face and the title "J'aime rire", I love to laugh.

Nine-year-old Jules left this drawing of a happy face and the words, J'aime rire, I love to laugh. ( ABC News: Barbara Miller )

Next to the drawing a packet of coloured pencils lies, a tribute to the veteran cartoonists executed because of their work.

The policeman whose cold-blooded murder was caught on film as he lay wounded on the street outside the Charlie Hebdo offices is also not forgotten.

"Je suis Ahmed", read some tributes, a modification of the phrase that has gone viral since Wednesday's attacks, "Je suis Charlie".

A man, a tourist I think, hands me his phone and asks me to take a photo of him at one of the memorial sites.

He walks closer to the flowers and drawings, bows his head and clasps his hands together in prayer. I take the photo for him.

He seems genuinely moved, but I feel a little uncomfortable with it, unsure whether I find it appropriate or not.

People hardly speak as they contemplate the tributes.

A woman takes a call on her phone, but moves away from the other onlookers and speaks in hushed tones.

Another woman in a business-like suit stands for a long time, every little while brushing tears from her eyes.

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All over this part of town businesses have put "Je suis Charlie" posters in their windows; they seem to be everywhere.

Move away from the centre and the slogan is harder to find.

In the traditionally working class, ethnically diverse 19th district, where one of the Kouachi brothers lived, I spot very few "Je suis Charlie" posters.

None of the hairdressers, the cheap clothing shops, the Asian restaurants, the cafes, or the travel agent specialising in trips to Mecca has chosen to put a "Je suis Charlie" poster in their window.

I find one finally in the window of a small art gallery.

The slogan might have gone viral, but it's not been universally adopted.

Now Parisians are gearing up for a rally the deputy mayor Patrick Klugman, himself Jewish, says will be a very important moment for his country.

It will be the end of the horror, he says, and the beginning of something new.

That at least is the bold hope.