The biggest security operation ever seen in Ireland was swinging into place tonight ahead of next week's historic State visit by the Queen.
More than 6,000 garda officers and Defence Forces troops are being deployed on the streets of Dublin while surveillance has been stepped up on known republican dissidents.
The massive operation is focusing on potential terrorist threats as well as plans for "a protest stunt" by breakaway factions opposed to the peace process, a garda source said.
Police chiefs are planning to outnumber about 200 known activists expected to stage demonstrations at key stop-off points during the four-day tour, including the Garden of Remembrance, which honours all who fought for Irish freedom, and Croke Park.
"They won't get anywhere near the Queen," said a source.
The garda special detective unit will be mingling among known republican sympathisers and activists, who won't be allowed through outer cordons anywhere the Queen is expected to get out of her car.
Anyone hoping to pass through high-security perimeters faces being searched although the garda said they would facilitate some protest.
"A reasonable protest will be allowed - a peaceful protest - but up to a point. Gardai are prepared, if necessary, to deal with any issues that arise," the source said.
Dublin's main thoroughfare, O'Connell Street, will be shut down throughout Tuesday on the day the Queen and the Duke of Edinburgh arrive at Baldonnel military aerodrome before beginning official engagements.
The Phoenix Park, where the royal visitors will be staying in the former home of the Guinness brewing dynasty Farmleigh, will also be closed, shutting off one of the main arterial routes into the capital.
Rolling closures will impact on several other main routes throughout Dublin as the Queen is transported to other engagements at Government Buildings, the war memorial at Islandbridge, Trinity College, the National Stud in Kildare and the Convention Centre.
"This will be the biggest security operation ever seen in the history of the State," said garda spokesman Superintendent John Gilligan.
The unprecedented four-day security operation is estimated to cost €30m.
Mr Gilligan insisted the city will remain open for business despite tight parking and transport restrictions on at least 30 roads.
There will be up to 4,000 gardaí on the streets at any one time in Dublin alone and more than 2,000 Defence Forces troops, made up of Army, Navy and Air Corps.
Potential threats from the air will be tackled by anti-aircraft guns, missile and radar systems deployed on the ground to liaise with seven PC-9 armed military planes patrolling Irish air space.
The Navy will also be combing off-shore waters and approach paths.
Soldiers will be deployed at key areas while the Army Ranger Wing special forces unit is on standby to take "direct action" in the event of a terrorist attack, a security source said.
Bomb disposal teams and engineer units are also on high alert.
"It's a huge operation in terms of policing, transport and logistics for the country. It is the biggest operation (ever)," Mr Gilligan said.
Commuters and motorists in Dublin city have been warned that from 6am tomorrow parking is banned on up to 30 streets until next Friday.
Elsewhere, the garda dive team, road safety chiefs and engineers have sealed thousands of manhole covers on VIP routes while 40km of crowd control barriers are in place around the capital.
Major national attractions like Dublin Zoo and the Japanese Gardens will temporarily close.
Security forces are expected to be particularly stretched on Wednesday night with the Europa League final taking place at the Aviva Stadium.
Transport chiefs warned there are no plans for additional bus, tram or train services to deal with the expected demand but that capacity can be increased if needed.
Conor Faughnan, Automobile Association (AA) spokesman, said he was not expecting chaos but warned the city will be more vulnerable to gridlock.
"It does make the city more vulnerable," Mr Faughnan said.
"We are down at least one artery from the north into the city, so evidently that will put pressure on the other arteries and if something goes wrong on one of those routes it will make things more problematic again."
The AA spokesman said an incident or security alert on one of the key alternative routes would be a major problem.
Garda chiefs said they will liaise with health chiefs to ensure access for emergency services and there is no question of any restricted access to hospitals and clinics.