POLICE on both sides of the Border were on high alert last night, with close-in surveillance on known terrorists as part of the security countdown to the visit of Queen Elizabeth tomorrow.
A number of dissident republican activists have already been arrested and questioned by officers, including the suspected chief of staff of the Real IRA.
A senior garda officer said: "We are now entering the crucial phase of what is the biggest security operation we have ever mounted, with 8,000 members involved.
"Contingency plans are in place to ensure that the schedule is not disrupted by potential attackers or troublemakers."
President Mary McAleese hailed the trip as an "extraordinary moment" in the history between the two countries, as unprecedented steps were taken to ensure the historic visit runs smoothly.
Five-hundred troops last night ringed the perimeter of Fortress Baldonnel as the military aerodrome on the edge of the capital went into lockdown for the arrival of the royal flight.
The soldiers took up position securing the base and will remain there until tomorrow afternoon, while nearby access roads were cordoned off.
Inside Baldonnel, military personnel from an Istar surveillance company went into action on light tactical armoured vehicles, fitted with sophisticated equipment.
A Giraffe air defence system is also on standby, with another located at Farmleigh in the Phoenix Park, where the queen is due to spend the three nights of her stay here.
Specialist search teams from the gardai and the Army have also been sweeping key location and the sites are then secure and kept sterile.
Gardai also brought forward plans to clear the streets around the Garden of Remembrance at Parnell Square in Dublin city centre as a result of indications that protesters intended to turn the first stop on her itinerary into a flashpoint.
Officers began erecting barriers around the square and North Frederick Street from Saturday morning and by yesterday the area had been restricted to all road traffic, with access limited to pedestrians.
A small group of 70 demon-strators, mainly drawn from the socialist republican group eirigi, staged a protest in the city centre yesterday afternoon, but were well-policed and dispersed without trouble.
However, bigger turnouts are expected at protests staged by various outfits in the coming days and while gardai said all peaceful demonstrations would be policed normally, officers are determined they will not be allowed to move close to the queen or her entourage or attempt to stage a propaganda stunt before the world's media.
More than 30 streets around the city were cordoned off over the weekend with parking restrictions remaining in place for the visit in areas including the north and south quays, Nassau Street, Pearse Street, Dame Street, Thomas Street, James's Street, Gardiner Street, Mountjoy Square and Conyngham Road.
Rolling restrictions will be implemented in other venues as the queen's motorcade travels down the N7 for a visit to the National Stud in Kildare town and Gilltown Stud outside Kilcullen on Thursday.
She will travel by helicopter on Friday to the Rock of Cashel and Coolmore Stud in Tipperary on Friday and spend two hours in Cork before flying home from there on the Royal BAe 146 craft.
More than 5,000 manholes, culverts and drains have been checked and sealed in the four counties and lamp posts have also been examined as potential hiding places for guns or explosives, while cyclists have also been warned not to leave their bikes chained to posts or railings.
Gardai and Army bomb disposal teams dealt with a number of hoax calls over the weekend as well as a pipebomb in Limerick and an incendiary device in Dublin's south inner city.
But the two explosive devices were planted by criminals and were not linked to visit of Queen Elizabeth.
The arrests on both sides of the Border, while not directly linked to the visit, were seen as a clear message to the dissidents that they face an immediate crackdown if there is trouble.
It is likely that the dissident terror groups will select a target in the North if they attempt to carry out an attack.
Four thousand gardai will be on duty throughout the visit at any given time, while up to 2,000 Defence Forces personnel are also either involved or on standby for callout.
The cost of the entire operation for Queen Elizabeth and next week's overnight visit by US President Barack Obama will be close to €30m.
QUEEN Elizabeth's arrival in Dublin tomorrow will be heartily welcomed by the overwhelming majority of this country's citizens. It symbolises, to quote British Prime Minister David Cameron, "the strong partnership and friendship" between Ireland and the United Kingdom.