Queen visits Ireland, London investigates bomb threat (UPDATES) (VIDEO)

Britain's Queen Elizabeth began a four-day tour of Ireland Tuesday, marking the first visit to the country by a British monarch.

In a significant gesture, she laid a wreath at the Republic of Ireland’s Garden of Remembrance, a site dedicated to the people who fought for Irish independence.

Earlier she was formally welcomed to the country by President Mary McAleese at her home in Dublin's Phoenix Park. The Queen arrived at Baldonnel military airbase, just outside Dublin, wearing an outfit of emerald green before switching into a white outfit after lunch, BBC reports.

A strict security clampdown meant that the streets of Dublin were largely empty. A pipe bomb found on a bus bound for the city on Monday had been defused by the Irish army.

On the eve of the historic visit, a bomb threat forced the Mall, an avenue leading to Buckingham Palace, in London to shut down.

Security sources in London told the Christian Science Monitor that Irish militants opposed to peace with Britain also warned of a bomb in central London.

"A bomb threat … has been received relating to central London today. The threat is not specific in relation to location or time," London police said in a statement Monday.

The caller reportedly knew a code word, which lent credibility to the threat.

The four-day trip will be the first visit to Ireland by a British monarch in a century and the first since independence.

Irish police have up to 4,000 people involved in security operations, BBC reports. It is the country's largest ever security operation, states Al Jazeera.

The Queen's itinerary includes events at Trinity College Dublin, the National War Memorial Gardens in Islandbridge, Dublin Castle and the highly symbolic Croke Park stadium.

Former Prime Minister Sir John Major, who worked on the Northern Ireland Peace Process in the early 1990s, said the Queen's visit would be a boost for the relationship between the UK and Ireland.

"If you're abroad and people talk about the Queen, they mean our Queen and I think the symbolism of her visiting Ireland – given the history of the past – will be seen as a very big event and absolutely pivotal event in building an even better relationship in the future," he told BBC.

Despite a 1998 peace deal, violence by dissident Republicans has been increasing in Northern Ireland and a militant group called the Real IRA told the Queen she was not welcome on Irish soil.