Swansea City say they will investigate after boos were heard from outside their ground while players took the knee before their draw against Reading.
Players from both teams made the gesture - as is customary at most clubs - in support of Black Lives Matter.
But as the empty Liberty Stadium fell silent, the sound of boos and horns blowing outside could be heard.
South Wales Police confirmed officers were at "a planned protest" which a "small group of people attended".
A South Wales Police statement said: "We have designated football liaison officers who work with clubs throughout the force area, including Swansea City, to help prevent and deter crime and anti-social behaviour.
"Officers engaged with protest organisers and attendees in order to remind them of their obligations under the current Covid-19 legislation.
"We will continue to work with the club and review any footage from the protest."
Swansea have not made any comment, but say they will look into what happened.
When asked about the incident after the game, Swansea head coach Steve Cooper said: "I can't really make a comment on it. I heard some noises but I couldn't tell what it was.
"Let's find out exactly what it was, the reasoning and rationale behind it, and I will give you a view of it then.
"But in terms of taking the knee, we are a club that's continuing to commit to it.
"We did have some discussions with the (club's) leadership team when there were some topical things going a couple of weeks ago, when some fans returned to stadiums.
"We made a joint decision that we wanted to continue with taking the knee."
Boos and the sound of horns blowing were clearly audible while players took the knee and for the first couple of minutes of the game.
"I was aware, but I didn't understand what was going on so I've got no comment," said Reading manager Veljko Paunovic.
No fans have been inside the Liberty since March because of coronavirus restrictions.
Swansea's fellow Championship club Millwall said they were "dismayed and saddened" after some fans booed players taking a knee at the start of their game against Derby earlier this month.
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