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.

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.