At least five police cars and an ambulance descended on the central London site at about 8pm this evening following reports of a stabbing.
The victim - believed to be a man in his 20s - was rushed to hospital but his condition is not yet known.
A cordon has been placed around the scene blocking off nearby bars and restaurants, while several police cars have blocked off the road.
Images from the scene show an air ambulance after it landed in nearby football pitches to attend to the incident.
And unconfirmed reports suggest a gang on motorbikes armed with a machete were involved in the attack.
TWITTER A man in his 20s was injured in the incident
One Twitter user said: "Hearing there were people on motorbikes with machetes going after people in Russell Square.. if you have loved ones round there check on em!"
Another added: "Massive incident at Russell Square! HUGE @metpoliceuk presence!"
One onlooker tweeted: "Footy finished early due to paramedic heli landing on pitch. Stabbing 200yards away. 20 odd youths chased. Brunswick square gardens."
It is believed to have occurred in the Bunswick Square Gardens area, near Russell Square tube station.
One man has been arrested on suspicion of Grevious Bodily Harm (GBH).
TWITTER A cordon has ben placed around the scene blocking off nearby shops
A Metropolitan Police spokesman said: “Police were called at approximately 19:45hrs on Friday, 9 June to reports of a number of people on mopeds involved in a disturbance in the vicinity of the Brunswick Centre, WC1.
“Officers, London Ambulance Service and London’s Air Ambulance attended and found a man, believed aged in his 20s, suffering a stab wound.
“He was taken to a central London hospital for treatment – condition awaits.
“One male – no further details – was arrested at the scene on suspicion of GBH."
But the police have confirmed the incident is not believed to be terror-related.
Britain has been on high alert since two Islamist militant attacks killed 30 people in Manchester and London in the last few weeks.
The terror level remains at severe, which means an attack is considered to be imminent.
TWITTER Police confirm the attack is not terror related
The attack comes almost a year after an American woman, 64-year-old Darlene Horton, was killed in a mass stabbing in the square.
Five others were knifed in the attack before suspect Zakaria Buchan, 19, was arrested.
He was charged with murder and attempted murder, and in February this year admitted the offences - although he said he did not know why he decided to carry out the horrific attack.