A night of torched vehicles and shop looting sparked in Dublin after a knife attack outside a school was of an "extraordinary" level unseen in decades, police said on Friday.

The violence started when a group broke through a police cordon Thursday in the area where three young children and a women who was caring for them were injured in a knife attack.

Groups went on to torch busses and trams and loot shops in one of Dublin's most famous throughfares, O'Connell Street.

Garda Commissioner Drew Harris told a press conference in the Irish capital on Friday that multiple Irish police officers were injured in a running battle with the group that stormed the crime scene in Dublin on Thursday night.

He said that one officer received a serious injury, with "numerous other members injured" as missiles were thrown at them.

"What we saw last night was an extraordinary outbreak of violence," Harris said. "These are scenes that we have not seen in decades."

Harris said "all lines of inquiry" are open to determine the motive for the knife attack.

Harris said 34 people were arrested after "huge destruction" by the "riotous mob" with 13 shops significantly damaged or subjected to looting.

A police cordon was set up around the Irish parliament building, Leinster House, late on Thursday night, amid concerns that the violence could spread.

- 'Motivated by hate' -

Irish Prime Minister Leo Varadkar said that protesters who battled police and looted shops were motivated by "hate" and brought "shame on Ireland".

"Those involved brought shame on Dublin, brought shame on Ireland and brought shame on their families and themselves," Varadkar told reporters.

For his part, Harris blamed a "complete lunatic faction driven by far-right ideology" for the disorder.

"We have a complete lunatic hooligan faction driven by far-right ideology, and also then this disruptive tendency engaged in serious violence."

Harris said calm was restored in the city shortly after midnight.

Irish Justice Minister Helen McEntee said the scenes of disorder were "intolerable" and that a "thuggish and manipulative element must not be allowed to use an appalling tragedy to wreak havoc".

"We will not tolerate a small number using an appalling incident to spread division," she said.

Some protesters carried signs reading "Irish Lives Matter" and waved Irish flags through a neighbourhood home to a large immigrant community.

One protester told AFP that "Irish people are being attacked by these scum."

Ireland has been facing a chronic housing crisis, with the government estimating that there is a deficit of hundreds of thousands of homes for the general population.

Widespread dissatisfaction has fed into a backlash against asylum seekers and refugees, and far-right figures have promoted anti-immigration sentiment at rallies and on social media with claims that "Ireland is full".

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