The Los Angeles-based Simon Wiesenthal Center on Monday voiced outrage over a Croatian rock concert by a popular ultra-nationalist singer that featured multiple "fascist" invocations. The Sunday evening Zagreb concert, which was attended by 60,000 people including Croatian members of parliament and the Croatian ministers of science, education and sports, turned into a massive fascist demonstration, the organization's chief Nazi hunter and Israel director Dr. Efraim Zuroff said. Thousands of people attending the rock concert by the singer "Thompson" shouted the infamous Ustasha regime's salute of "Za dom spremni," while numerous participants came wearing Ustasha uniforms and symbols, the center said in a press release. "Under the current circumstances, I believe that the time has come to prohibit public concerts by those who write songs of nostalgia for [the] Jasenovac [concentration camp] and inspire the show of Ustasha symbols, which constitute open and blatant incitement against all the minorities in Croatia," Zuroff wrote in a Monday letter to Croatian President Stjepan Mesic. "I believe that only if someone of your stature and outstanding anti-fascist credentials will lead the efforts to combat this ugly wave of revived fascism, can this extremely dangerous new trend be stopped before it engulfs Croatia," he continued. The Ustasha regime ran a puppet government after the Nazi invasion of Yugoslavia in 1941. During their four years in power, the Ustasha carried out a Serb genocide, exterminating over 500,000 people, expelling 250,000 and forcing another 250,000 to convert to Catholicism. The Ustasha also killed most of Croatia's Jews, 20,000 gypsies and many thousands of their political enemies. After the war, most of the Ustasha leaders escaped to South America and Spain. During the collapse of Yugoslavia in the 1990s, there was a certain resurgence of Ustasha symbols coinciding with the ethnic hatred that remained after the wars.