The Mediterranean city is in mourning after a van ploughed into crowds in its busy Las Ramblas boulevard on August 17, followed just hours later by a vehicle attack in the seaside resort of Cambrils further south.
Fifteen were killed in the carnage and Spanish Prime Minister Mariano Rajoy has called on Spaniards to turn up in force to show their "love" and solidarity with Catalonia where the rampages took place. The march also comes after knife attacks on soldiers and police in Brussels and London on Friday evening.
Also present will be King Felipe VI, the first Spanish sovereign to take part in a demonstration since the monarchy was re-established in 1975 after the death of dictator Francisco Franco.
'Love' for Barcelona
The king will attend to "express his love for the people of Barcelona, of Cambrils, of Catalonia," Rajoy said Friday.
"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 1st.
But he and Catalonia's separatist president Carles Puigdemont made a show of unity in the immediate aftermath of the Islamic State group-claimed attacks on August 17th and 18th.
They will both attend the march that kicks off at 6:00pm local time (1600 GMT).
Already on Friday evening, thousands of people marched against terror in Cambrils, shouting "no tinc por", which means "not afraid" in Catalan - as defiantly shouted by locals last week after the attacks.
The slogan of the Barcelona demonstration will also be "no tinc por."
Those who tended to the victims last week will be given pride of place at the top of the procession.
These 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 city hall's head of social emergencies who helped people who were lost or who couldn't find their loved ones.
Over the following days, she and her colleagues gave families psychological support when they were given terrible news, and also helped others like doctors and firefighters.
"There are a lot of people who are suffering," she said, adding that even psychologists themselves had struggled.
For her, the march will help "recognise the work of people who have been on the front line."
Ode to peace
Saray Gomez, an 18-year-old who works at a flower stall right next to where the van ended its murderous rampage, said it was important "to give a message of unity and peace."
"And it's important to distinguish between Islam and jihadists, because Muslims are the first to be affected."
Thousands of red, yellow and white flowers - the colours of Barcelona - will be distributed to protesters.
The march will end on the Plaza de Catalunya near Las Ramblas where two violoncellists will interpret the Cant dels ocells (the song of the birds), a traditional Catalan song which has become a symbol of peace.
In 1961, late composer Pau Casals had played it at the White House in a rejection of the Franco regime.
By Laurence Boutreux and Anna Cuenca