Catalan police reported that finance minister Cristóbal Montoro and the Catalan PP chief Alicia Sánchez Camacho had just left a meeting in the town of Vilanova i la Geltrú when a group of people began to throw objects at their car.
Police detained one man for striking an officer with a stick and have not ruled out further arrests, according to national daily 20 Minutos.
The Catalan branch of Spain's ruling Popular Party (PPC), made an official report of the incident to the regional Interior Ministry claiming that the protestors "were carrying out an 'escrache'" — a type of protest which involves verbally harassing politicians on the street — when they began to throw bottles and rocks at the car.
The driver of the vehicle had to look for an alternative exit route, leading to confrontations between riot police and the demonstrators.
Speaking to Spanish public radio, Sánchez Camacho said: "During the entire election campaign, the PP in Catalonia has faced permanent demonstrations from anti-eviction campaigners, aggression, and acts from anti-system protestors but we have always had a guarantee of safety from the Mossos d'Esquadra (the Catalan Police force)."
"This afternoon the machinery broke down and we are going to ask who is responsible for the failure of the operation," she added.
"We were completely defenceless, surrounded by attackers," she said.
In the PPC's official report, they condemned the police action as "insufficient" and "negligent" and called upon the head of the region's police force, Manel Prat, to increase the presence of riot police given the magnitude of incidents which are occurring.
The protestors were reported to have been carrying anarchist and pro-Catalan independence flags plus symbols in support of Basque political prisoners.