Amid a huge security lockdown, Queen Elizabeth II arrived in the Republic of Ireland on Tuesday, starting the first visit by a British Monarch since Ireland's independence in 1921.

The monarch, accompanied by her husband Prince Philip, landed at Baldonnel Aerodrome in the afternoon. She was greeted by British Ambassador Julian King, Ireland's Ambassador to the UK Bobby McDonagh and Ireland's deputy prime minister Tanaiste Eamon Gilmore. However, the royal visit has been marred by security threats.

A bomb was found in the luggage compartment of a bus last night near Dublin which was made safe by the Irish police by carrying out a controlled explosion. According to an Irish defence forces spokesman, an army bomb disposal squad had made safe a viable improvised device early on Tuesday at Maynooth in County Kildare. It was on a bus and by the time our team was called in, the bus was evacuated and parked at a bus stop, the spokesman said.

Another package was found at a tram stop in Inchicore, Dublin, and was made safe by the Irish army. However, it was later confirmed to be a false alarm. Police in London have also destroyed an abandoned suitcase in a controlled explosion in Trafalgar Square.

Despite the potential danger, the Queen did not cancel her historic visit, said Irish Prime Minister Enda Kenny.

All the devices follow a coded threat issued by Republican dissidents to Scotland Yard on Monday. However, the security plans were not altered, since the telephone threat contained a recognized code word, but without any specific time or place, they said.

Gerry Adams, the leader of Sinn Fein said that the royal visit could provide 'a unique opportunity' for establishing mutual respect and equality on both sides of the Irish Sea, although, his party has previously denounced the visit as 'premature and insensitive'.