The Queen has laid a wreath at Dublin's Garden of Remembrance where Ireland commemorates those who died resisting British rule - as 21 people were arrested amid protests at the royal visit.
The British national anthem was played as the monarch stood beside Irish President Mary McAleese during the first official royal visit in 100 years, and the first since independence.
The visit took place amid the biggest security operation ever mounted in the state with more than 8,500 police and armed soldiers on the streets of the capital.
The Garden of Remembrance opened in Easter 1966 to mark the 50th anniversary of the 1916 Rising when a revolution against British rule was launched.
The wreath-laying ceremony took 15 minutes, but has been viewed as a symbolic act of reconciliation between Britain and the Irish Republic.A group of dissident republicans, opposed to the peace process in Northern Ireland, were held back by riot squads, and around the city, 100 protesters chanted slogans and burnt the Union flag.
Police have confirmed that 21 people were arrested on public order offences.
President McAleese condemned dissident republicans who were opposing the trip as "the tail end of a very old, tired, failed culture of trying to resolve political problems through paramilitarism".The monarch, dressed in emerald green and accompanied by the Duke of Edinburgh, landed at Baldonnel aerodrome at around midday.
She was greeted by British Ambassador Julian King, Ireland's Ambassador to the UK Bobby McDonagh and Ireland's deputy prime minister, Eamon Gilmore.
The Queen was then escorted down a red carpet, lined on either side by members of the Irish Air Corps guard of honour, before accepting flowers from eight-year-old Rachel Fox, from south Dublin.
A motorcade then took the 85-year-old to meet Mrs McAleese at her residence in Phoenix Park, Aras an Uachtarain.Once there, she signed a visitors' book dating back to 1802 and planted an Irish oak sapling outside.
Her actions mirrored those of the last British Queen to visit - Queen Victoria - who planted a redwood in 1861.
The start of her visit was marred, however, by the discovery of two suspicious packages.
The first device - a pipe bomb - was found on Monday night in the luggage compartment of a bus on the outskirts of Maynooth, near Dublin.Irish police carried out a controlled explosion to make that safe in the early hours of the morning.
The second, found at a tram stop in Inchicore, Dublin, was made safe by the Irish army, but was later confirmed to be a hoax.
The royal visit to Ireland coincided with the 37th anniversary of the Dublin-Monaghan bombings.
Sinn Fein President Gerry Adams visited the memorial to the dead in Dublin, and said the Queen's trip could have a potentially positive outcome if British files on the bombings were released.
Survivors and families of the bereaved wrote an open letter to the Queen, requesting the information be made public.The final stop on the first day of the visit was at Trinity College, Dublin, where the Queen and Prince Philip viewed the Book of Kells.
The ninth century gospel manuscript is kept in the Old Library, along with the college's royal charter which was signed in 1592 by Queen Elizabeth I.
Former Irish prime minister Brian Cowen formally invited the Queen to Ireland last year.
He declared that relations between Britain and Ireland have been transformed by the peace process.
"The vast, vast majority of the people of Ireland wanted to see this day come," he said.