Brazilian midfielder Taison was sent off after he reacted to alleged racist abuse during Shakhtar Donetsk's 1-0 home win against Dynamo Kyiv.
Taison, 31, made a gesture towards the Dynamo fans and kicked the ball at them before the referee took the players off the pitch in the 77th minute.
When they returned five minutes later, Taison was shown a red card.
Shakhtar said both Taison and fellow Brazilian Dentinho were targeted and the Dynamo fans were warned twice.
Video on social media shows both Brazilians in tears.
In a statement after the game, Dynamo said they would cooperate with any investigation into the incident.
"It's terrible when such things happen in football," said Shakhtar midfielder Marcos Antonio after the Ukrainian Premier League match. "The game needs real fans instead of the people who come to the stadium and behave like that, not respecting the athletes.
"We need to work on this because such episodes upset us all. Dentinho, Taison, the other guys feel just terrible right now."
Team-mate and goalkeeper Andriy Pyatov added: "Everyone is somewhat shocked by what has happened in the stands today. I've seen that for the first time ever. I've never thought that people can behave like that. It doesn't flatter anyone.
"We discussed the accident with the Dynamo players in the players' tunnel. They also have representatives of other nationalities in the team. I saw that [Dynamo player Gerson] Rodrigues was also shocked, because this, as it turns out, concerned him as well. The referee had said he would stop the game if this happened again."
Shakhtar manager Luis Castro said: "I want to support absolutely everyone suffering from racism and the guys who have suffered today.
"Any manifestations of racism are absolutely unacceptable. It was, is, and will be a disgrace to everyone. Together, we must fight it - every day, every minute and every second."
In a club statement, Shakhtar said they "categorically oppose any manifestations and forms of racism, racial discrimination, xenophobia, and any related intolerance".
Shakhtar added: "There can be no justification of those who insult other people because of their skin colour, religious, or political beliefs. This kind of behaviour is unacceptable in any civilized countries and football leagues.
"Such humiliating accidents cause pain and disappointment to the large football family of FC Shakhtar. We condemn this kind of behaviour of the audience, we will always voice the support and protect our players. We are calling on football authorities and clubs to stop racism in the stadiums."
In a tweet after the game, Dynamo wrote "#NoToRacism".
"We consider hasty the allegation of a predetermined guilt of the fans, when the investigation into the circumstances of this case has not yet begun," Dynamo said.
"We look forward to the start of the investigation, ready to assist it comprehensively and, if proven guilty, will take all possible steps to further isolate these individuals not only from football but also from society.
"We look forward to the release of photos and videos that could shed light on all the circumstances of the incident."
Last week in Italy's Serie A, Brescia's Mario Balotelli kicked the ball into the crowd and threatened to walk off the pitch after being abused in a 2-1 defeat at Verona.