Ellen Pompeo has slammed the ''aggression'' of police officers during the Black Lives Matter protests in the US.

The 'Grey's Anatomy' star attended a protest for the cause last week, and has now spoken out to condemn the ''force'' she has seen used by police trying to control the protests, as she claims there was more ''ammunition'' present than compared to any protest she has been to in the past.

In a video posted to her Instagram Story, Ellen said: ''I've been to other protests. I've never seen police with machine guns [there]. I've never seen the National Guard with machine guns. We're at a march for black lives, and we're here to talk about the fact that they matter. And there's machine guns - big ones - and aggression.

''They're on rooftops, they're in cars, they're in tanks, and I haven't ever seen that show of force and that show - not that it's as forceful as a lot that we've seen, but, I mean, we're in the middle of the day here.

''At other protests, I've never seen that amount of ammunition. So, I'm just wondering why that is: Why at the Women's March there's no machine guns. Why at Pride marches there's no machine guns. But the Black Lives Matter march, machine guns everywhere. I don't know. You tell me.''

The Black Lives Matter protests began across America in late May following the death of unarmed black man George Floyd, who was killed during an arrest when a police officer - who has since been charged with second-degree murder - knelt on his neck for almost nine minutes.

Following the protests, several stars have joined the cause or donated funds to help those involved, and rallies have spread to other countries including the UK, where 'Star Wars' star John Boyega recently addressed the crowd in a moving speech.

He said: ''I need you to understand how painful this s**t is. I need you to understand how painful it is to be reminded every day that your race means nothing and that isn't the case anymore, that was never the case anymore.

''Black lives have always mattered. We have always been important. We have always meant something. We have always succeeded regardless. And now is the time. I ain't waiting.''