The English Football Association has launched investigations after returning Millwall and Colchester United fans booed as players took the knee.
Fans were welcomed back to The Den for the first time since March but a section of those lucky enough to get a ticket for their English Championship match against Derby County booed as players took the knee just before kick-off.
Colchester United's League Two match against Grimsby Town at the Colchester Community Stadium was also overshadowed by some supporters booing as players there made the same gesture.
"The FA can confirm that investigations are underway into crowd-related incidents at both The Den and JobServe Community Stadium on Saturday 5 December 2020," reads the official statement released by the governing body on Monday afternoon.
"Observations have been sought from all of the relevant parties and they will have until Thursday 10 December 2020 to provide their respective responses."
Players in the UK, Ireland and around the world have been taking the knee to show solidarity with the Black Lives Matter movement, after the killing of George Floyd earlier this year.
The gesture was started in the U.S. in 2016 where the then-San Francisco 49ers quarterback Colin Kaepernick took the knee during the pre-match rendition of the national anthem at a game to protest against police brutality and racism and many athletes in America followed suit.
Colchester owner Robbie Cowling says fans who want to boo players taking a knee are not welcome at the club.
"The game is being talked about because of the ridiculous actions of a few who decided to boo our players whilst they took the knee before kick-off," said Cowling, in a statement.
"It is undeniable that black and other ethnic minority people are still the victims of racism, and the black footballers and staff at Colchester United feel that they are in a position of privilege that has been fought for through the blood and sweat of their ancestors. A position that in 2020 the average black person is still not afforded.
"Those taking the knee, and supporting the taking of the knee, not only show their willingness to support the drive to eradicate racial oppression but force it to be a talking point even when it's uncomfortable.
"Maybe those that booed on Saturday might now understand what this gesture means to our club and will at the very least remain silent during future games whilst the players continue to take the knee before each kick-off.
"Alternatively, they should just stay away from our club because anyone that still wants to boo now that I have explained the purpose and importance of the taking of the knee is not welcome at our club.
"I will be happy to refund anyone for the remaining value of their season permit if that is the reason they feel they can no longer attend our games."
A statement from Millwall expressed regret over the incident at The Den.
"Millwall Football Club was dismayed and saddened by events which marred Saturday’s game against Derby County at The Den.
"The club has worked tirelessly in recent months to prepare for the return of supporters and what should have been a positive and exciting occasion was completely overshadowed, much to the immense disappointment and upset of those who have contributed to those efforts."