Moussa Marega, a Malian international playing for Portuguese side FC Porto, walked off the pitch on 16 February after he was apparently racially abused by opposition fans in a match against Vitoria de Guimaraes.
Team-mates and opponents tried to restrain Marega as he headed for the sidelines to no avail. It is the first case of a walkout in a high-profile football game.
It is the latest racism-related incident at a football match that has made headlines in various European countries in recent months.
Another shocking case took place in Scotland: police announced on 10 February that a 12-year-old boy has been charged in connection with racist chants against Alfredo Morelos - a Colombian international playing for Rangers.
The boy cannot be identified for legal reasons.
On 31 January, the UK Home Office had released figures quoted by the Guardian newspaper showing that racial incidents in English football had risen more than 50% in the 2018/19 season.
More than 150 football-related racist incidents were reported to the police last season, double the number registered three seasons before.
The situation in England is particularly concerning because the Premier League - one of the sport's richest and most-watched competitions - has built up a reputation nationally and abroad for a stronger anti-discriminatory stance than its European counterparts.
The year of 2019 ended with crowd trouble at the Manchester derby on 7 December - a Manchester City supporter was arrested after allegedly making monkey noises towards Manchester United midfielder Fred.
"It's deplorable that this kind of thing is happening, but it's even more worrisome when it takes place in England, where there has always been such an effort to stamp racism out," Gilberto Silva, a former Arsenal midfielder and 2002 World Cup winner with Brazil, told the BBC.
Back in October, black players representing England had been targeted by opposition fans in a Euro 2020 qualifier against Bulgaria.
That led to a media outcry against racism in support of the England players, but now critics say the country needs to put its own house in order.
"We used to be famous for tolerance, inclusiveness, all those things. We are in danger of losing that," says Iffy Onuora, who has played for and managed professional league teams in England, and now works for the Professional Footballers' Association (PFA).
But the country is far from alone in suffering a resurgence in racism - other major incidents across Europe this season include a controversial red card given to Brazilian player Taison during a Ukrainian league match.
He was sent off last November for reacting to abuse during Shakhtar Donetsk's 1-0 home win against Dynamo Kiev.
Taison made an offensive gesture towards the Dynamo fans and kicked a ball towards them.
In the Netherlands, first and second division clubs did not play the first minute of matches on the weekend of 23 November as a protest against racism - a week earlier a match had been suspended for 30 minutes after Den Bosch supporters racially abused Excelsior forward Ahmad Mendes Moreira.
There was also controversy in Spain on 25 January when Athletic Bilbao player Inaki Williams was subjected to monkey chants by a section of the home crowd in a game at Espanyol.
Referee Jose Sanchez took no action, despite calls from the Athletic player, and did not mention the incident in his match report either - although a few days later Espanyol announced it was banning 12 supporters for life.
But the lack of action by the referee led to an outcry, especially because matches have been stopped for other reasons.
In December, a match between Rayo Vallecano and Albacete was abandoned over chants aimed at Roman Zozulya about his alleged far-right leanings, for example.
And the racism extends outside the football stadium too. In Turkey, Trabzonspor filed a police complaint following online racial abuse directed by Fenerbahce fans towards Nigerian international John Mikel Obi.
Abuse is not restricted to the top level: in Germany, Hertha Berlin's under-16 team walked off the pitch on 15 December during a game away at Auerbach in which players reported racial abuse - this time from opposition players.
"There is a lot that needs to be done. Start with a shift in mindset, to move from complacency and outrage to knowing there is a contagion of racism across European football," says Piara Powar, the executive director of Football Against Racism in Europe (Fare), a network of anti-discrimination NGOs.
Some campaigners believe the rise in the number of incidents is being fuelled by the perceived leniency in the punishment for racist offenses.
The Fare network criticised the European football governing body UEFA after it decided to punish the Bulgarian Football Association with just a $97,000 fine and a ruling that its national team should play one home game behind closed doors after its fans abused English players.
The incident marked the third time that Bulgarian supporters had been punished for behaving in this way.
World governing body Fifa has rules allowing tougher sanctions.
'Failure to act'
Players or officials guilty of racism are liable to a minimum ban of 10 games. Clubs or international teams whose supporters engage in discriminatory behaviour can suffer points deductions or even be disqualified or relegated from a competition.
However, none of the more extreme measures has ever been applied.
"The contagion in Europe is characterised by factors such as the rise of far-right political movements but also by failure to act by football associations," Powar adds.
"They should move away from action against individuals to actions against teams."
In an interview with the British Daily Mirror newspaper, UEFA president Aleksander Ceferin admitted that "more needs to be done" to combat racism.
Mussa Okwonga, a Berlin-based British writer and journalist, told the BBC that complacency was part of the issue. "Some people argue that things are not as bad as they were before," he says.
"The problem with that argument is that it assumes progress is inevitable. The question should probably be how badly football is losing the fight against racism," he adds.
Daniel Kilvington, a senior lecturer at Leeds Beckett University, sees the rise in incidents across Europe as the consequence of the changing political landscape in the continent.
"When countries lurch through social and political unrest, racism and xenophobia are rarely far behind," he wrote in October.
Italy is one of the countries where the ground has shifted.
The latest edition of a traditional yearly survey carried by polling firm SWG found that 55% of Italians said that racist acts were "justified at least some of the time".
Incidents of discrimination in Italian football have proliferated in the last few years. In November, Brescia striker Mario Balotelli tried to leave the pitch after receiving abuse from the Hellas Verona fans.
He was dissuaded by team-mates and opponents alike.
In Italy too abusive behaviour can be found at all levels in the game.
Before his death last month, Italian sociologist and writer Mauro Valeri recorded more than 80 cases over two seasons in youth football, some involving children as young as 12.
"Italy has never regarded itself as a multi-ethnic country, and while other countries have seriously addressed racism in football, Italy never has," Valeri told the UK's Guardian newspaper in September.
Further dismay came in early December when national newspaper Corriere dello Sport ran the headline "Black Friday" above a story featuring two black players, Inter Milan's Romelu Lukaku and Roma's Chris Smalling.
When Lukaku and Smalling criticised the headline, the newspaper rejected accusations of racism and claimed the headline was "innocent" and "made poisonous by those who have poison inside them".
Then, a few weeks later, came the Italian league's anti-racism campaign: on 16 December it unveiled posters depicting the faces of three monkeys.
"The world is looking at us and we just can't get it right," Esquire Italia magazine commented.
Fare executive director Powar says that the lack of diversity in dugouts and boardrooms has a profound affect on responses to racist incidents. Although black players are numerous in all major European leagues, there is a major lack of representation among managers and directors.
In the 2018 World Cup, only one of the 32 national team managers was black.
A Fare 2014 study found that across European football only 0.6% of senior governance positions were held by ethnic minorities, and only 0.4% of senior executive roles were filled by ethnic minorities.
"It is important to address diversity as a matter of urgency. Where are the ethnic minority leaders? Where are the black managers?" Powar asks.
"They don't exist and it's a problem."
Campaigners and players have also warned of a rise in online abuse. Anti-racism organisation Kick It Out reported 22,000 discriminatory social media posts directed at players and teams who took part in the 2016 European Championships.
Last April, England-based professional players staged a 24-hour social media boycott in order to demand stronger action in curbing online abuse.
In 2018 Brazil and Manchester City midfielder Fernandinho temporarily ruled himself out of the national team after being on the receiving end of racial abuse on Twitter following Brazil's World Cup defeat by Belgium.