As he left the soccer field after a club match in the eastern German city of Halle on March 25, the Nigerian forward Adebowale Ogungbure was spat upon, jeered with racial remarks and mocked with monkey noises. In rebuke, he placed two fingers under his nose to simulate a Hitler mustache and gave a Nazi salute.

In April, the American defender Oguchi Onyewu, playing for his professional club team in Belgium, dismissively gestured toward fans who were making simian chants at him. Then, as he went to throw the ball inbounds, Onyewu said a fan of the opposing team reached over a barrier and punched him in the face.

International soccer has been plagued for years by violence among fans, including racial incidents. But FIFA, soccer's world governing body, which is based in Zurich, said there had been a recent surge in discriminatory behavior toward blacks by fans and other players, an escalation that has dovetailed with the signing of more players from Africa and Latin America by elite European clubs.

This "deplorable trend," as FIFA has called it, now threatens to embarrass the sport on its grandest stage, the World Cup, which opens June 9 for a monthlong run in 12 cities around Germany. More than 30 billion cumulative television viewers are expected to watch part of the competition, and Sepp Blatter, FIFA's president, has vowed to crack down on racist behavior during the tournament.

The issue has been included on the agenda at FIFA's biannual congress, scheduled to be held this week in Munich. A campaign against bigotry includes "Say No to Racism" stadium banners and television commercials, and team captains will make pregame speeches during the quarterfinals of the 32-team tournament.

Players, coaches and officials have been threatened with sanctions. But FIFA has said it would not be practical to use the harshest penalties available to punish misbehaving fans - halting matches, holding games in empty stadiums and deducting points that teams receive for victories and ties.

Players and anti-racism experts said they expected offensive behavior during the tournament, including monkey-like chanting; derisive singing; the hanging of banners that reflect racist beliefs; and perhaps the tossing of bananas or banana peels, all familiar occurrences during matches in Spain, Italy, Eastern Germany and Eastern Europe.

"For us it's quite clear this is a reflection of underlying tensions that exist in European societies," said Piara Powar, director of Kick It Out, an organization against racism in soccer based in London. He said of Eastern Europe: "Poverty, unemployment, is a problem. Indigenous people are looking for easy answers to blame. Often newcomers bear the brunt of the blame."

Yet experts and players also said they believed the racist behavior would be constrained at the World Cup because of increased security, the international makeup of the crowds, higher ticket prices and the prestige of the event.

"Racism is a feature of many football leagues inside and outside Europe," said Kurt Wachter, project coordinator for the Football Against Racism in Europe, an international network of organizations. He said he expected most problems to occur outside stadiums, where crowds are less controlled. "We're sure we will see some things we're used to seeing. It won't stop because of the World Cup."

Germany has one of the world's lowest rates of violent crime. Still, an immigrant group called the Africa Council said it would publish a "No Go" guide for nonwhites during the World Cup, particularly for some areas of eastern Berlin and for surrounding towns in the state of Brandenburg.

Chancellor Angela Merkel has warned that "anybody who threatens, attacks or, worse, kills anybody because of the color of his skin or because he comes from another country will face the full force of the law."

The Bundesliga in Germany is one of the world's top professional soccer leagues and has not experienced widespread racism. Incidents involving racial abuse of black players are more prevalent in semiprofessional and amateur leagues in Eastern Germany.

After making a Nazi salute, which is illegal in Germany, Ogungbure of Nigeria was investigated by the authorities. But a charge of unconstitutional behavior against him was soon dropped because his gesture had been meant to renounce extremist activity.

"I regret what I did," Ogungbure said in an interview by telephone from Leipzig. "I should have walked away. I'm a professional, but I'm a human, too. They don't spit on dogs. Why should they spit on me? I felt like a nobody."

Gerald Asamoah, a forward on Germany's World Cup team and a native of Ghana, has been recounting an incident in the late 1990s when he was pelted with bananas before a club match in Cottbus. "I'll never forget that," Asamoah said during a television interview. "It's like we're not people." He has expressed anger and sadness over a banner distributed by a rightist group that admonished, "No Gerald, You Are Not Germany."

Cory Gibbs, an American defender who formerly played professionally in Germany, said there were restaurants and nightclubs in Eastern Germany - and even around Hamburg in the West - where he was told, "You're not welcome," because he was black.

"I think racism is everywhere," said Gibbs, who will miss the World Cup because of a knee injury. "But I feel in Germany racism is a lot more direct."

Racist behavior at soccer matches is primarily displayed by men and is fueled by several factors, according to experts: alcohol; the perceived "us versus them" threat of multiculturalism in societies that were once more ethnically homogeneous; the difficult economic transition of East European countries since the fall of the Berlin Wall; and crude attempts to unnerve opposing players during bitter, consuming rivalries.

Other observers say the soccer stadium in Europe has become a communal soapbox, one of the few remaining public spaces where spectators can be outrageous and where political correctness does not exist and is even discouraged.

"Nowhere else other than football do people meet someplace and have a stage for shouting things as an anonymous mass," said Gerd Dembowski, director of an anti-racist organization called Floodlight, based in Berlin. "You can shout things you would never say in your normal life, let out your frustrations."

Not all the misbehavior can be traced to fans or to Europe. Players and coaches have also been transgressors.

Luis Aragonés, Spain's World Cup coach, was fined in 2004 after making racial remarks about the French star Thierry Henry. In March, in the Brazilian league, the defender Antonio Carlos was suspended for 120 days, and four additional matches, after an incident in which he shouted "monkey" at an opposing player who is black.

But it was an incident in Spain on Feb. 25 that galvanized anti-racist sentiment and prodded FIFA into taking a tougher stand against bigoted behavior. That match, in Zaragoza, was temporarily halted in the 77th minute by the referee, who threatened to cancel the remaining 13 minutes after Samuel Eto'o, the star forward for Barcelona, was subjected to a chorus of racial taunts. Eto'o threatened to leave the field, but his coach and teammates persuaded him to continue, and last month Barcelona won the European Champions Cup.

Eto'o, who was voted European player of the year this spring, has become one of the sport's most outspoken players on the subject of racism.

"I'll continue to play," Eto'o, whose national team, Cameroon, did not qualify for the World Cup, said last week through his agent. "I'm not going to give up and hide and put my head down. I'll score goals against the teams whose fans are making rude noises."

Under pressure to curb what it acknowledged was an increase in racist incidents, FIFA announced in late March a stricter set of penalties that would apply for club and national team matches. The sanctions would include suspensions of five matches for players and officials who make discriminatory gestures, fines of $16,600 to $25,000 for each offense and two-year stadium bans for offending spectators. It also said teams, which receive three points in the standings for a victory, would have three points deducted on a first offense by misbehaving players, officials or fans.

Blatter, the FIFA president, told reporters that the three-point deduction for abhorrent fan behavior would apply during the World Cup, then backed away from his comments in April. Blatter declined to comment for this article. And it remains unclear exactly what penalties will be levied against World Cup teams for offensive behavior by fans, coaches and players.

Nicolas Maingot, a FIFA spokesman, said World Cup sanctions would be made public later. But in an e-mail response to questions, he said: "Only racist abuses in the field of play will be punished. For fans, it will be impossible, due to the multinationality of the audience.

"In other words, it would be impossible to identify from which side would potential racist abusers come."

Critics counter that spectators are supposed to have their names on their tickets, so identifying offending fans should be relatively easy.

Onyewu, the American defender who was punched by an opposing fan in Belgium, said the man was identified through an anonymous tip and was barred from attending matches for two years. Onyewu said he did not retaliate, because he believed that racist behavior reflected acts of a minority of fans.

"I'm anticipating a more professional environment in Germany because it's the World Cup," Onyewu said. Even so, he said, although anti-racist efforts could restrict public behavior, "that's only helping the exterior."

He added, "The interior mind thinking, you can't really change that."