Manchester United said they "will take the strongest possible action" they can after defender Ashley Young was racially abused on Twitter.
Young, who was captaining the side, was targeted as United suffered a 3-0 second-leg defeat at Barcelona in their Champions League quarter-final on Tuesday to go out 4-0 on aggregate.
United said the club "utterly condemns" the "racist social media comments".
The club added it was "working to identify individuals involved".
Anti-discrimination charity Kick it Out said: "Yet another black player, this time Ashley Young, is targeted with racist abuse on social media after tonight's Champions League game.
"Again, we're left asking Twitter the same question - when will you take serious action to tackle the rampant discrimination on your platform?"
"We do not comment on individual cases for privacy reasons," a Twitter spokesperson said.
"Our abusive behaviour policy prohibits behaviour which attempts to harass, intimidate or silence another person's voice and we take strong enforcement action if violations are found.
"Progress in this space is tough, but we are deeply committed to improving the health of the conversation on the platform.
"To this end, we are now suspending three times more abusive accounts within 24 hours after receiving a report than this time last year. We'll continue building on this work to prioritise the safety of our users."
Accounts identified in this case as violating Twitter's abusive behaviour policy have been prevented from posting again until the offending tweets have been removed.