The UK faces demands today to halt all exports of tear gas and riot gear to Donald Trump's America.
Campaigners and Labour said the government must suspend licences for the export of "riot control projectiles and equipment" to the US.
Exports must only resume once the UK has established if riot gear may be used against people protesting George Floyd's death, Labour said.
Shadow International Trade Secretary Emily Thornberry made the demand in a letter to her government counterpart Liz Truss.
She wrote: "At a time when Donald Trump is gearing up to use the US military to crush the legitimate protests taking place across America over the murder of Black civilians, it would be a disgrace for the UK to supply him with the arms and equipment he will use to do so.
"If this were any other leader, in any other country in the world, the suspension of any such exports is the least we could expect from the British government in response to their actions.
"Our historic alliance with the United States is no reason to shirk that responsibility now."
It comes after tens of thousands of US people defied curfews to take to the streets on Tuesday for an eighth night of protests.
George Floyd died after a white policeman pinned his neck under a knee for nearly nine minutes in Minneapolis on May 25.
The incident reignited the explosive issue of police brutality against African Americans.
But as protests erupted, President Donald Trump vowed to crack down on what he called lawlessness by "hoodlums" and "thugs," using National Guard or even the US military if necessary.
Priests and protesters were tear gassed yesterday to clear the way so the President could pose for photos at a church near the White House.
Roxie Washington, mother of Floyd's 6-year-old daughter, Gianna, told a news conference he was a good man.
"I want everybody to know that this is what those officers took from me....," she said, sobbing.
"Gianna does not have a father. He will never see her grow up, graduate."
The officer who knelt on Floyd, 44-year-old Derek Chauvin, has been charged with third-degree murder and second-degree manslaughter. Three other officers involved were fired but not yet charged.
Labour called on the government to "suspend all existing licences and halt the issue of any new licences for the export of riot control projectiles and equipment to the United States", until an assessment of their use is carried out.
According to the Campaign Against Arms Trade pressure group, there have been £18m of ammunition sales, £800m in small arms licences and £2m in licences for other goods, mostly shields, from the UK to the US since 2010.
But many of these will have been for the military and there is no breakdown of how much was used by police, or what for, according to CAAT.
There are also not clear figures on how much tear gas or "crowd control ammunition" was sold from the UK to the US because there is an "open" licence, allowing unlimited exports, CAAT said.
Andrew Smith of Campaign Against Arms Trade said: "The brutal and racist police violence we have seen over recent days has been absolutely appalling.
"So has the reckless and totally irresponsible escalation from the President and his colleagues.
"These arms sales should never have taken place and the government must ensure that they do not happen again.
"This kind of equipment is always repressive and it can be deadly. That is why there must be an urgent investigation into what weapons have gone over and if any of them have been used against protesters.
"Silence in the fact of such injustice is compliance."
Amnesty spokesman Oliver Feeley-Sprague added: "Given the evidence emerging from numerous US cities, there's a very real risk of UK-manufactured tear gas or rubber bullets being used against George Floyd protesters in dangerous and highly inappropriate ways."
Downing Street has failed to directly condemn the President over his response to protests.
Instead Boris Johnson's official spokesman said this week: "The violence we’ve seen in the US over recent days is clearly very alarming.
“People must be allowed to protest peacefully.
“As the Foreign Secretary said yesterday, the footage of George Floyd’s death is deeply distressing and our thoughts are with all those who’ve been affected.
“You’ll be aware that the suspect has been charged and there’s a federal review under way. We would hope and expect justice to be done.”
He added: “The reported arrests and incidents [involving journalists] are very concerning. Journalists all around the world must be free to do their job and to hold authorities to account without fear of arrest or violence.”
A Government spokesman said: "We take our export responsibilities seriously and assess export licence applications in accordance with strict licensing criteria."