Henry Nowak died after the knife attack in the southern England city of Southampton in December.
His killer Vickrum Digwa, a 23-year-old Sikh man, was on Monday sentenced to life in prison, having lied to police at the time that Nowak had assaulted him.
In police bodycam footage, Nowak is seen lying on the street saying "I've been stabbed" and "I can't breathe" while an officer responds, "I don't think you have, mate".
Prime Minister Keir Starmer said there were "serious questions" to answer, including how "allegations of racism informed or fed into the decision-making in that particular case".
"It is impossible to watch that footage and not appreciate that those questions absolutely have to be answered," Starmer told reporters.
Judge William Mousley acknowledged in court on Monday the case had stirred racial tension across Britain.
Nigel Farage, whose anti-immigration Reform party leads opinion polls, said it was an example of the rights of ethnic minorities trumping those of white British people.
"The fear of being called racist was greater than dealing with Henry Nowak's murder," he said in a statement.
"We should respond to this with pure cold rage."
A protest later on Tuesday outside the Southampton police station drew a few hundred people chanting "I can't breathe", including anti-immigration activist Tommy Robinson. Other protests have been advertised for this week.
Digwa stabbed Nowak with a knife he said he was permitted to carry due to exemptions for Sikhs to have ceremonial daggers.
When the police arrived, Digwa said his turban had been knocked off and he had an injury to his eye. Nowak's family called his treatment by police "inhumane and degrading" but in a statement outside court, his father said his death should not be "used to create further division, hatred or tension".
Reform leader Farage sought to draw parallels with the 2020 killing of George Floyd in the United which sparked the Black Lives Matter movement. Floyd had said "I can't breathe" as a police officer knelt on his neck for several minutes.
Nowak died shortly after being handcuffed by police. Once they realised he was injured, they uncuffed him and started CPR.
The police force, Hampshire Police, has apologised.
In a separate statement on Tuesday, the force said one of the officers involved in the arrest had resigned, while three others were being treated as witnesses in the investigation.
A spokesperson for the attorney-general's office said it had received multiple requests to consider raising Digwa's 21-year minimum sentence, adding that law officers had 28 days from sentencing to make a decision on any changes.
