10,000 gardaí and soldiers are to be deployed as part of the largest security operation ever in Ireland for the forthcoming visits of Britain’s Queen Elizabeth and US President Barack Obama.

Surveillance has been increased on known republican activists and al-Qaeda sympathisers, some of whom have already been arrested and questioned.

Security for the two visits is estimated to cost more than €30m.

The Defence Forces will be responsible for policing the country's seas and airspace.

Gardaí in charge of the security operation say the places to be visited will not be locked down and restrictions will be minimised and temporary.

A security camera with a panoramic view is being installed 18m above Dublin city centre.

Thousands of manholes are being sealed; 40km of barriers are being put in place, and national attractions like Dublin Zoo and the Japanese Gardens will temporarily close. The VIP routes have been checked and residents along them have been interviewed.

Surveillance has been increased on dissident republican activists and those suspected of links with Middle Eastern terrorist organisations.

Ten people have so far been arrested.

Gardaí and the Defence Forces believe there is no active al-Qaeda unit based in Ireland and are focussed on local sympathisers who fundraise and provide logistical support such as safe houses and false documentation.

Queen Elizabeth will arrive at Baldonnel military airbase on Tuesday and the Defence Forces have deployed Giraffe Radar systems on camouflaged vehicles to monitor the airspace, along with surface-to-air missiles, surveillance cameras and PC 9 planes to patrol the skies.

Protests have been planned by organisations such as Republican Sinn Féin. Gardaí say peaceful protests will be facilitated - they will be kept away from the VIP routes and anyone suspected of breaking the law will be arrested.