U.K. authorities pleaded for calm following a knife attack in Belfast, Northern Ireland,?that led to the arrest of a Sudanese national and prompted protest calls from the far right.

Video circulating online of the late Monday attack appeared to show the assailant stabbing at his victim’s head before being beaten away by passers-by, one of them using a hurling stick. With the victim hospitalized in serious condition, Prime Minister Keir Starmer condemned the incident as “horrific” and “abhorrent.”

The incident is the latest in a string of racially-charged attacks providing fodder to the hard and far right in the U.K.. Anti-immigration activist Stephen Yaxley-Lennon, known more often as Tommy Robinson, posted video of the Belfast incident online, referring to the attacker as an “invader.” He also amplified calls for Tuesday-night protests across the U.K.