Pedestrians, believed to be anti-racism demonstrators, blocked the M6 motorway following a protest in Coventry.
A group of people walked between junctions 3 and 2 in Warwickshire, leading to the closure of the road, police said.
It came on the day of a series of demonstrations sparked by the death of George Floyd in Minneapolis last month.
Also on Sunday, about 1,000 people gathered at a Black Lives Matter protest in Wolverhampton.
The Coventry protest saw hundreds of people kneel for a minute's silence in the city centre.
"It felt like a moment of collective prayer led by young people," BBC reporter Navkiran Mann tweeted from the scene.
But shortly after 18:00 BST a group of people walked on to the M6 to the north of the city, bringing traffic to a standstill
The road was reopened after about an hour when the group left at junction 2, but police warned of continuing tailbacks.
In Wolverhampton, actor Shereen Walker, one of the organisers, said the "peaceful, static protest" at West Park aimed to promote unity because "racism is alive".
The gathering coincided with a protest outside the US Embassy in London.
Large numbers of people congregated outside the embassy in Nine Elms, near Vauxhall, while other protests are taking place across the UK.
At the event in Wolverhampton's West Park, protesters fell silent and knelt for eight minutes, 46 seconds to represent the time prosecutors said the police officer had his knee on George Floyd's neck.
Ahead of the protest, Ms Walker said prayers would remember "the victims of police brutality in America" and their families.
"I do believe that this is a heart problem, a problem inside of ourselves that needs to be addressed," she said.
"Let's appeal to one another to learn to love, recognise our responsibility to one another and activate change. Racism is alive."
Mr Floyd's death triggered a wave protests across the United States as well as anti-racism demonstrations around the world.
Footage of his arrest shows a white police officer, Derek Chauvin, kneeling on Mr Floyd's neck while he was pinned to the ground.
The officer was dismissed and has been charged with murder.