Violence broke out in Dublin this afternoon following republican protests over the visit of Queen Elizabeth to Ireland.
Gardaí said 21 arrests were made for alleged public order offences.
Twenty people are due before Cloverhill District Court tonight. One other person was released without charge having received a caution at Pearse Street Garda station, a statement said.
About 100 people attacked gardaí at the junction of Dorset Street, Eccles Street and Hardwicke Street with rocks and bricks early today. A number of fires were lit, and there were running battles with gardaí.
Protesters from Republican Sinn Féin and the 32 County Sovereignty Movement originally held a demonstration at North Frederick Street, adjacent to the Garden of Remembrance before the gardaí began pushing them back towards the Dorset Street junction.
They were joined by onlookers who began burning bags of rubbish and taking bricks from building sites close by to throw at uniformed gardaí and members of the riot squad.
The junction, which was originally open to traffic, was closed. A number of cars and coaches were caught up in the clashes.
Earlier, a crowd of around 30 supporters of the socialist republican group Éirigí gathered at the Spire on O’Connell Street to stage a protest. They sat in the northbound carriageway of O'Connell Street and chanted slogans and beat bodhrans while some of the protesters set fire to a union flag.
They were surrounded by gardaí, who prevented others from joining the protest.
Soon afterwards, a crowd of about 100 people approached the Spire from Henry Street, to be met by ranks of uniformed gardaí, who forced them back using crowd control barriers.
They retreated down Henry Street where they were joined by the group from the Spire and the crowd moved to Moore Street for a rally. The crowd was addressed by speakers from Éirígí before making its way up Moore Street towards Parnell Street.
About 200 people staged a sit-down protest at the junction of Parnell Street and Parnell Square. A Garda spokesman said a number of people were arrested for public order offences and were taken to Store Street Garda station.
Seperately, Sinn Féin released hundreds of black balloons into the air as the Queen laid a wreath at the Garden of Remembrance this afternoon.
A protest organised by the Irish Anti-War Movement against the Queen's visit passed off without incident this evening.
The protest sought to highlight the role of the Queen as the head of the British armed forces.
About 100 people turned up for the protest which began at the GPO and ended with the release of black balloons outside the Central Bank.
Chairman of the organisation Richard Boyd Barrett said their protest was not anti-British, but was to highlight the role of the British Crown forces in Afghanistan and Iraq.
He described the visits of Queen Elizabeth and US President Barack Obama as a "circus" similar to that organised by the Roman emperors to distract the masses from wars and destruction.
The protesters were addressed by Joan Humphreys, a British woman who lost her grandson Private Kevin Elliott in Afghanistan in 2009.
She said a million Afghans had been mourning following the conflict which started in the country in 2001 and which she described as a "criminal war".
She said there were wars that were just, such as the Spanish Civil War, but the conflicts in Afghanistan and Iraq were not among them.