Nine Real Sociedad football fans were injured, one seriously, in central Rome late on Wednesday, allegedly after being violently attacked by hardcore “ultra” Lazio supporters.

According to reports in Italian media, the Spanish fans were among a group of about 70 allegedly set upon by what appeared to be about 80 Lazio ultras, who were armed with clubs, chains and knives and hammers, outside a pub on Via Leonina in the Monti district of the Italian capital. Security in the city has been upped before the Europa League match at Rome’s Stadio Olimpico on Thursday night.

Three of the people injured were in hospital with stab wounds. One, aged 23, is in a serious condition, according to reports. Three others were discharged after treatment and three more were reportedly examined for their injuries in a hospital emergency unit before leaving.

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Italian police have seized the weapons used in the attack and are examining CCTV images to identify those responsible. In a video clip purporting to be of the incident recorded by a Monti resident and posted on social media, a gang, dressed in black and believed to be the Lazio ultras, are running along a street and can be heard shouting “we kill everyone”.

La Repubblica newspaper reported that the attack happened as the Spanish fans were having a drink at the Irish pub Finnegan.

It is unclear whether Real Sociedad ultras, a term that originated in Italy but is used globally to define groups of hardcore, and often violent, football supporters, were among the group. Police are working to establish whether the attack was planned by the suspected Lazio ultras, who are on the extreme right of the political spectrum, against their left-wing counterparts.

Walter Verini, a senator and coordinator of an anti-mafia parliamentary committee, told the LaPresse news agency the attack was “further confirmation of the severity of a problem which, unfortunately, concerns parts of a fanbase.

“Not all fans are criminals. It seems to be understood that this armed violence is somehow also linked to delusional political extremists, in the case of far-right Lazio.”

The Rome mayor, Roberto Gualtieri, said the attacks were “shameful and unacceptable”. He said: “Rome strongly rejects these scenes of crazy violence which offend the city, its citizens and the spirit of sport. I trust that the investigations by police will soon shed light on those responsible because the Italian capital does not want and does not deserve to be remembered for such barbarism.”