DUBLIN - Queen Elizabeth II embarks on the first visit on Tuesday by a British monarch to the Republic of Ireland since it won independence, surrounded by a massive security operation.

In a highly charged trip aimed at reconciliation following the peace established in British-ruled Northern Ireland, the queen will undertake an historic four-day state visit to the republic.

Irish security forces are on full alert for the visits of the 85-year-old monarch and US President Barack Obama in the space of a week.

A 10,000-strong force will be deployed in the country's biggest-ever security operation amid threats from dissident republican paramilitaries opposed to any official British presence on Irish soil.

Mainstream republicans Sinn Fein, the political wing of the now-defunct Irish Republican Army (IRA), are also hostile to the visit by the queen and her husband Prince Philip.

Sinn Fein leader Gerry Adams on Saturday called the trip "premature and insensitive".

However, the ground-breaking visit has been welcomed by most Irish people, with a recent survey finding 81 percent in support.

Irish Prime Minister Enda Kenny has described it as "symbolic of the end to years of division and the start of a brand new relationship".

Ireland gained independence from Britain in 1922, but no British monarch has visited since the queen's grandfather King George V in 1911.

The itinerary does not sidestep difficult issues, with Queen Elizabeth visiting some of the most sensitive sites in Anglo-Irish relations.

After being welcomed by President Mary McAleese, the pair will lay wreaths at the Garden of Remembrance in Dublin, dedicated to those who died fighting for Irish freedom.

On Wednesday, the queen visits Croke Park stadium, where in 1920 British forces shot dead 14 people in a reprisal attack. It was a key moment in the Irish independence struggle.

The program also includes lighter moments, with visits to the Guinness brewery, the national stud and a traditional market.

However, despite the official bonhomie, the security forces are taking no chances.

Fears were heightened when a Catholic policeman was killed by a car bomb in Northern Ireland last month, in an attack blamed on dissident republicans opposed to the peace process.

The clampdown includes some 8,000 police and 2,000 soldiers, with many Dublin streets barricaded off.

Police on both sides of the Irish border have arrested several dissident republican suspects ahead of the visit. Some fear such groups will attempt to hijack the visit with an attack.

A state visit would have been unthinkable during the Troubles, the three decades of sectarian strife between Northern Ireland's Catholics and Protestants.

Agence France-Presse