The postings were condemned by the Anti-Defamation League, the New York-based advocacy group that last week released its first global survey on anti-Semitism, which showed that 29 percent of Spanish adults harbor prejudicial stereotypes about Jews. “The sheer number and intensity of anti-Semitic hatred unleashed via Twitter in Spain is alarming and outrageous,” Abraham H. Foxman, the organization’s national director, said Thursday.

The anti-Semitic outbursts came the same week the Barcelona soccer club dismissed an employee of its museum after she was caught on video making monkey gestures toward an African player during a game between Llagostera and Racing Santander on Sunday. Llagostera said the police would investigate the matter and banned the woman from its stadium. That episode followed one last month in which someone threw a banana at Dani Alves, a Brazilian member of the Barcelona soccer team, during a match against Villarreal.

Taken together, the outbursts lay bare an undercurrent of prejudice in Europe that has persisted through generations. Social media appears to have fueled the hostilities, while also serving to counter them.

Esteban Ibarra, the president of the Movement Against Intolerance, a Spanish advocacy group, said it had identified 1,500 websites, pages or blogs in Spain that promote racism or anti-Semitism, compared with 300 to 400 five years ago. He attributed the rise, in part, to the growing political success of extremists in countries like Hungary and Greece.

“The fact that Spain doesn’t have an extreme-right party with an institutional presence doesn’t mean that we don’t have extremists who have been encouraged and coordinate with others in Europe and make their presence most felt in sports,” Mr. Ibarra said. “What we’re seeing in cases like Maccabi and Dani Alves is that the groups of ultra sports fans are themselves infiltrated by neo-Nazis.”