Spanish police said on Wednesday they had arrested three suspected members of a jihadist cell in Barcelona and one in Madrid after one of them tried to buy an assault rifle for a potential attack.
A fifth suspected cell member was already in prison serving an unspecified sentence.
Officers found three machetes and ammunition in the house of the group's leader, identified only as "Sheikh", who had been previously arrested in Turkey in 2016 when trying to enter a conflict zone to join the Islamic State, police said in a statement.
He later travelled to several countries such as Malaysia, Tanzania and Algeria, where he continued to recruit for the Islamic State, the police said.
Spanish police detected his presence in Barcelona in March 2021, where he had contacted a gang of young Algerians who robbed tourists and who later "showed strong signs of radicalisation".
The activities of jihadist groups remain a priority for police in Spain after Islamist militants used a van to plough into a crowd of pedestrians in Barcelona's Las Ramblas boulevard in August 2017 and carried out a follow-up attack in another municipality, killing 16 people in total.