What sparked the recent violent protests in the United Kingdom?

LONDON -- A man wrapped in a red-and-white English flag raised his hand in a Nazi salute in the town center of Leicester, England, as a line of police officers looked on and a crowd of counterprotesters shouted out their collective disapproval.

"Off our streets, Nazi scum," the counterprotesters chanted, according to a video of the incident posted on social media and verified by ABC News. The man saluting during the Aug. 3 rally appeared to hold his hand high throughout the duration of the 23-second clip.

Similarly racist, combative and at-times-violent scenes have played out on streets throughout the United Kingdom for more than a week. London's Metropolitan Police has described the ongoing far-right rallies as a "national critical incident."

"We will not tolerate this on our streets," the Met said in a statement. "We will use every power, tactic and tool available to prevent further scenes of disorder."

Dozens of additional rallies led by far-right groups were expected to be held Wednesday evening in London and elsewhere, the police said. Counterprotests were expected at many of them. Patrols were being increased in London in advance of the rallies, police said in a statement late Tuesday.

"We arrested more than 100 people in central London disorder last week and we will not hesitate to arrest hundreds more if they take to the streets intent on fueling violence," the Met said.

The unrest follows the deaths of three girls, who were stabbed in a "ferocious" attack during a July 19 dance event in Southport, a seaside town, according to police.

A 17-year-old was arrested and charged with murder, police said. The suspect was from Banks, a coastal village in Lancashire, and was born in Cardiff, Wales, police said.

The Crown Court released the suspect's name after a judge ruled it could be released despite his age. Although the suspect was born in the United Kingdom, online rumors spread calling into question his immigration status, police said.

"This recent activity is a clear lesson in how important it is to counter any misinformation posted online or on social media," Shane O'Neill, Leicester Police's chief superintendent, said in a statement.

Some of those who've attended the rallies have voiced concerns over immigration policies in the wake of a the stabbing deaths. Keir Starmer, the newly elected prime minister, described their actions as "far-right thuggery."

"We will ensure those responsible will feel the full force of the law," Downing Street said in a statement. "We will not tolerate attacks on mosques or our Muslim communities. The full force of the law will be visited on all those who are identified as having taken part in these activities."

Starmer said Downing Street had called for "swift action" against violent protesters, and the Crown Prosecution Service already charged and brought several rioters into court.

One 37-year-old pleaded guilty Tuesday to violent disorder after he "hurled missiles at police officers ... following widespread unrest in Hull," prosecutors said in a press release. Another 18-year-old from Bolton was sentenced Tuesday to two months in jail after pleading guilty to damaging two police cruisers during the unrest, prosecutors said.

Several arrests were also made in Leicester, where two protests were held in the city center on Aug. 3, including one 35-year-old woman who was later charged with racially or religiously aggravated intentional harassment and assault for allegedly beating an emergency worker, the Leicester Police said in a statement.

Along with the hundreds who've been arrested at rallies, others were arrested for "online offenses," Starmer's office said Tuesday.

"If you provoke violent disorder on our streets or online, you will face the full force of the law," Starmer said.

ABC News' Camilla Alcini contributed to this report.