Red, yellow and white flowers - the colours of Barcelona - will be distributed to protesters and the slogan for the march will be "no tinc por" -- Catalan for "Not afraid".
The Mediterranean city is in mourning after a van ploughed into crowds on Las Ramblas boulevard on August 17, followed hours later by a car attack in the seaside town of Cambrils.
Fifteen were killed in the carnage and Spanish Prime Minister Mariano Rajoy has called on Spaniards to turn out in force to show their "love" and solidarity with Catalonia, where the rampages took place.
Europe has been shaken by a spate of deadly Islamist violence with an increasing number of low-tech attacks using vehicles or knives - sometimes both - as weapons.
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Last evening, a man was shot dead in central Brussels after stabbing a soldier while shouting "Allahu akbar" (God is greatest) and shortly afterwards another man with a large knife attacked police in London as they tried to arrest him outside Buckingham Palace.
"There, with all of Catalan society and all of Spain... we will once again give a clear message of unity and condemnation of terrorism, and of love for the city of Barcelona," he added.
The warm comments contrast with Rajoy's earlier criticism of Catalan leaders, with whom he has been at loggerheads over their plans to hold an independence referendum on October 1.
But in the aftermath of the attacks, which were claimed by the Islamic State group, he and Catalonia's separatist president Carles Puigdemont made a show of unity. Both will attend the march, which begins at 6:00 pm (local time).
Those who tended to the victims last week will be given pride of place at the top of the procession.
They include security forces, emergency workers, residents and shop owners in the Las Ramblas avenue and taxis who took people for free.
People like Montse Rovira, the 53-year old city hall's head of social emergencies, will also march at the head of the procession. Her service helped people who were lost or who couldn't find their loved ones.
"There are a lot of people who are suffering," she said, adding that even psychologists themselves had struggled.