Three young children were among five people injured in a knife attack in Dublin on Thursday that sparked riots in the city centre and which police have not yet ruled out any motive over, including whether it could be terror-related.

Public transport was suspended and patients advised not to travel to a nearby maternity hospital unless absolutely necessary after clashes between riot police and anti-immigrant protesters, who arrived at the scene of the attack beside the main thoroughfare of O'Connell Street.

A double decker bus was burned to the ground in front of the Daniel O'Connell statue at the head of the street and windows were smashed at a nearby Holiday Inn hotel and McDonalds restaurant. A Footlocker store was looted.

"They are disgraceful scenes. We have a complete lunatic, hooligan faction driven by far right ideology engaged in serious violence," Police Commissioner Drew Harris told reporters after deploying 400 officers to restore public order.

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A police car was also burnt out.

Such rioting is almost unprecedented in Dublin. There are no far right parties or politicians elected to parliament, but small anti-immigrant protests have grown in the last year. The government is reviewing security around parliament after a recent protest trapped lawmakers inside.

Harris said all lines of inquiry related to the attack remained open, contradicting a senior officer who had earlier told reporters that police were satisfied the incident was not terror-related.

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"I'm not going to speculate any further in respect of a terrorist motive. Until we're sure what the motive is, we have to keep an open mind as to why this happened," he said.

A five-year-old girl was receiving emergency treatment after sustaining serious injuries in the stabbing. A man in his late 40s, also being treated for serious injuries, was arrested by police who said they are not looking for any other suspect.

Police said it appeared the man attacked a number of people on Dublin's Parnell Square shortly after 1330 GMT. Members of the public intervened at an early stage of the attack.

A woman in her 30s was also being treated for serious injuries while the two other children, a five-year-old boy and six-year-old girl, suffered less serious injuries. The boy has been discharged from hospital.

"There was complete and utter pandemonium, women wailing, men screaming and crying," Anthony Boyle, 31, an IT consultant who lives on the road and was passing by told Reuters.

Sow Division

The scene was still sealed off shortly before 1800 GMT when a group of around 50 anti-immigrant protesters briefly broke through a police barrier. Some shouted "get them out" and one kicked the wing mirror off a police car. Another was draped in an Irish flag.

A larger crowd then began throwing objects and firing fireworks at riot police in helmets and shields, once they were deployed, and the rioting spiraled out of control. The crowds began to disperse around 2100 GMT.

Police would not comment on the nationality of the detained man.

Net migration among Ireland's 5.3 million population rose to its second highest level since records began in the 12 months to April and around 100,000 Ukrainian refugees have arrived since Russia's invasion, among the highest per capita in the EU.

"There is a group of people, thugs, criminals, who are using this appalling attack to sow division," Justice Minister Helen McEntee told reporters.

"Every force possible is being used by the Gardai (police) to restore order. This will not be tolerated."