At a small event held on Baghdad's central Tahrir Square and organised by the Avaaz global campaigning group, activists put up posters and handed out postcards carrying the messages.
"My heart and thoughts are with all the victims, we will never forget you," read one message signed by Manon, from the French Riviera city of Nice, where a truck ploughed through a crowd on July 14, killing 84 people.
That attack was claimed by the Islamic State group, although the attacker's links to the group remain unclear.
The jihadist organisation, which has its roots in Iraq, also claimed responsibility for a July 3 suicide truck bomb attack in Baghdad's busy Karrada neighbourhood which the government said killed at least 323 people.
Dozens of suicide attacks have hit Baghdad this year alone, and organiser Ahmed Twaij said those killed in the Iraqi capital often attract little solicitude from the West.
"It's just a statistic," the activist said.
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"One hundred people dead, 200 people dead... they're not seen as someone's brother who just passed away, or someone's sister, someone's mother.
"Iraqi lives are just as important as lives in the West," said Twaij.
"So these are letters from around the world in a sign of solidarity towards the people of Iraq to show that... they are not alone."
Many of the messages were from Nice and Paris but others originated from a number of other countries, including the United States.
"People from every corner of the world have been hit by brutal terror attacks in recent weeks, but from Paris to Baghdad, Mogadishu to Kabul we stand united against these attempts to divide us," Luis Morago, Avaaz campaign director, said in a statement.
"Iraqis know all too well what can happen when we split apart and turn on each other," he said.
According to the Iraqi health ministry, 208 of the people killed in Karrada a month ago required DNA testing to determine their identities.