Inequalities have risen sharply in Spain more than any country in the OECD group of nations with the exception of Cyprus.
A report from Oxfam says the disparities have become greater in Spain since 2007 than even Greece or Mexico.
The Organisation for Economic Cooperation and Development encompasses 34 member countries, including many of the world's most advanced countries but also emerging economies such as Mexico, Chile and Turkey.
Oxfam said inequalities rocketed in Spain "almost 10 times more than the European average - and 14 times more than in Greece."
"In 2014, 29% of the Spanish population was at risk of poverty or social exclusion, 2.3 million more people than in 2008, exceeding the EU average by more than six percentage points."
The agency found that the wealth of the 20 richest people in Spain went up 15% last year, but the assets of the remaining 99% of people fell by the same percentage.
It pointed to "a combination of a huge salary gap with a backward tax system that doesn't much burden those who have the most."
"There has been a collapse in the average salary in Spain, with a 22% drop between 2007 and 2014-15," the group said, adding that the presidents of the 35 main listed companies "earn 158 times more than the salary of an average worker."
Oxfam said that tax evasion was rife in Spain.
"Investment from Spain to tax havens rose 2,000% in 2014," it said.
Rising inequalities was one of the prominent themes of the December general elections. Much criticism about the conservative Popular Party’s tough austerity programmes which included tax rises, labour reforms and harsh cut-backs in public spending.
