Antoni Tapies' works were sprawling, abstract oeuvres that sometimes featured discarded everyday materials and graffiti-like scrawls. He liked the letter X to be in them, and the number 4, for the four elements of nature.
Many may not have understood his work but his creativity in painting and sculpture made him one of the world's top contemporary art figures.
Tapies died Monday at age 88 after several years of poor health, leaving a major gap in the Spanish art world.
"Tapies is without a doubt Spanish art's most prominent figure of the second half of the 20th century," Reina Sofia museum director Manuel Borja-Villel wrote in El Pais newspaper.
Born in Barcelona in 1923, Tapies was one of Spain's main exponents of abstract and avant-garde art in the second half of the 20th century.
"Inheritor of the genius of the first vanguard movement, which had as its chief representatives Picasso, Miro and Dali, he was a constant presence in our country over the past 60 years," said Borja-Villel.
Borja-Villel was director of Barcelona's Tapies Foundation from its opening in 1990 until 1998. As a tribute to the artist, the foundation was opening its doors for free Tuesday and Wednesday. Long queues of people formed outside the center from early Tuesday.
"He was the most radically Catalan artist in his thinking, his expression and references and at the same time the most universal in his language and international projection," Catalan regional government president Artur Mas said in a statement.
Tapies _ who won many awards over the course of his career, including the 2003 Velazquez Prize, Spain's top art award _ was known for his large format works with thick smears of paint and crudely distorted figures. He also incorporated everyday objects such as buckets, mirrors and pieces of furniture into his work, as the appropriately named "Desk and Straw" (1970) and "Mattress" (1971) demonstrated.
Other notable works included "Gray Relief on Black" (1959) and "White and Orange" (1967), "Pants and Woven Wire (1973) and his famous 18-meter long sculpture "Sock."
In 1948, Tapies helped co-found the first Post-War Movement in Spain known as Dau al Set (The Seven-Spotted Die), which was connected to the Surrealist and Dadaist Movements.
The Encyclopaedia Britannica described him as having introduced contemporary abstract painting into Spain. It said his use of the impasto thick-paint technique lent his work a power and individuality comparable to the American Abstract Expressionist paintings and helped secure his international reputation.
Tapies was married to Teresa Barba Fabregas. The couple had three children.
The foundation said his family had asked for his funeral to be a strictly private affair, adding that a public wake would not be held.