FC Dallas defender Reggie Cannon has condemned the "disgusting" booing of players kneeling during the national anthem by some home fans before their match against Nashville on Wednesday.
Almost 3,000 fans were inside the Toyota Stadium, with supporters permitted at some Major League Soccer (MLS) games if local rules allow.
Some boos could be heard as players from both teams took a knee in protest against racial injustice.
"How disgraceful is that?" said Cannon.
"You got fans booing you for people taking a stand for what they believe in. Millions of other people support this cause and we discussed with every other team and the league what we're going to do and we've got fans booing us in our own stadium.
"I think it was absolutely disgusting."
The match, which Nashville won 1-0, was the first major league sporting event with fans since the MLS was suspended in March because of the global coronavirus pandemic.
During that time, protests have swept across the United States and worldwide following the death of George Floyd, an unarmed African-American man who died on 25 May after a white police officer knelt on his neck for several minutes.
Former Minneapolis police officer Derek Chauvin faces charges of second-degree murder and manslaughter, while three other officers were also sacked and charged with aiding and abetting murder.
"I'm sorry for our fans because we had someone chanting 'USA' when they don't understand what kneeling means," added 22-year-old USA international Cannon.
"They don't understand why we're kneeling - they can't see the reason, they just think we're the ignorant ones and it's incredibly frustrating.
"When we decided to kneel I knew it was going to happen. That should tell you something. I knew we were going to have some negative pushback from having a unified response over what's going on.
"That's a problem."
Cannon said Dallas' intention to kneel had been communicated to both Nashville and the league before the match.
He had asked for the US national anthem not to be played but the MLS has previously said it will be when fans are in attendance.
A person was reportedly escorted out of the stadium after throwing a water bottle at players as they were kneeling.
"It hurts me because I love our fans. I love this club and I want to see the support that the league has given us - that everyone has given us - from our fans," said Cannon.
"As soon as I heard that boo I tried to play on and I knew what to expect. We'll take it one day at a time and again we're unified in this response and everyone stands together."
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