"The Spanish King is not welcome in the Catalan countries," the banner read in English. It was accompanied by an upside-down portrait of the monarch.
The king and Prime Minister Pedro Sanchez are due to join families of the victims for the commemoration at the Plaza Catalunya, near Las Ramblas, Barcelona's main avenue where a jihadist ploughed through crowds in a white van, killing 14 people on August 17, 2017.
People gather ahead of the ceremony on Friday morning. Photo: AFP
During his escape, the 22-year-old Moroccan attacker also stabbed to death a young man before stealing his car.
Catalonia's main separatist civil society group, the Catalan National Assembly, on Twitter expressed "all its support to the activists who deployed this banner and spent the entire night defending freedom of expression".
Family members of the victims of the attacks in Barcelona and the nearby seaside resort of Cambrils -- where a woman was stabbed to death and several other people injured just hours after the Barcelona van rampage -- had asked for a "truce" in the political conflict over Catalonia's separatist drive on the one-year anniversary of the attacks.
The king had been jeered by Catalan separatists, who reject the monarchy and hope to set up an independent republic, when he joined a massive protest against terrorism in Barcelona shortly after the attacks.
Catalan separatists organisations will stage their own commemorations of the attacks on Friday so as to avoid appearing alongside the king, who adopted a hardline stance against Catalonia's separatist push last year.