"There is a high visibility police presence in Barnsley town centre this morning responding to a serious incident. More information will follow. Public are asked to be vigilant and report any concerns to 101," South Yorkshire police said.
The attacker has been arrested and is now in custody, one person has been injured, according to South Yorkshire police.
The Counter Terrorism Policing detectives have joined the investigation.
UPDATE: Please see below for update on #Barnsley town centre incident. One person is in police custody. If you see anything suspicious or hold information about this morning's incident, please call 101 quoting incident number 246 8/9/18 pic.twitter.com/dAlXzlaX9y— SouthYorkshirePolice (@syptweet) 8 сентября 2018 г.
Social media users have written numerous posts related to the incident.
According to unconfirmed reports, two more attackers armed with knives are on the run.
Serious incident in Barnsley with someone stabbing people. Our family are stuck in the market. Police have locked it down.— Beryce Nixon (@BeryceNixon) 8 сентября 2018 г.
A witness posted on Twitter that the woman was holding a "large knife" and was "trying to stab everyone near traders' stalls and shouting in Arabic."
The local shops and market are now fully functioning.