In unusually warm comments at a time of high tension between Rajoy and separatist leaders in Catalonia where the vehicle rampages took place, he praised the rapid work of the local police and spoke of the state's affection for the region.
The twin vehicle attacks in Catalonia last week -- a van rampage in Barcelona and a car attack in the nearby seaside resort of Cambrils -- killed 15 people and injured more than 120 others.
Rajoy called on "everyone to participate in tomorrow's protest (Saturday) in Barcelona" against extremism.
"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 said.
Rajoy will join King Felipe VI, Barcelona Mayor Ada Colau, Catalonia's President Carles Puigdemont and many others at Saturday's march in Barcelona.
Speaking to reporters, he said the king -- Spain's head-of-state -- would take part to "express his love for the people of Barcelona, of Cambrils, of Catalonia".
He also praised the rapid work of the region's Mossos d'Esquadra police squad in dismantling the terror cell behind the attacks.
Speaking after a cabinet meeting, he said the core nucleus of the terror cell had been "completely disarticulated in just 100 hours.
"That says a lot about the preparation and ability of the Mossos," he added, congratulating the Mossos.
He also extended his praise to the Guardia Civil and Spain's national police, in what appeared to be a bid to ease tensions between the country's security forces.
Earlier this week, the unions of the Guardia Civil and national police complained the Catalan force had left them out of the investigation for political reasons.
It also accused the Mossos of ignoring important information they had about suspects.
In their joint statement, they claimed that Catalonia's separatist executive had left them out in the cold because they wanted to project an image abroad of Catalan self-sufficiency ahead of a planned October 1st referendum on independence.
It is as yet unclear whether the vote will take place, as Madrid has declared it illegal.