The Irish army blew up a makeshift bomb found in a bus outside Dublin ahead of a historic visit by Britain's Queen Elizabeth on Tuesday, amid the biggest security operation ever mounted by the state.

The Queen's visit, the first by a British monarch since Ireland won independence from London in 1921, is designed to show how warm neighbourly relations have replaced centuries of animosity.
   
But the discovery of the bomb on a bus in a town outside Dublin, and a coded warning on Monday about a possible bomb in London, were stark reminders that a small minority remain violently opposed to continued British rule in Northern Ireland.

Britain's Foreign Office said the Queen's visit would still go ahead despite the bomb's discovery.

It was found in the town of Maynooth, 25 km (15 miles) from Dublin and blown up by an army bomb disposal unit in a controlled explosion, the military said. The remains of the device were handed over to police for investigation.   

Police later said the army's bomb squad was investigating a second suspicious device found at a Dublin tram station.   

Peace in Northern Ireland after decades of conflict has paved the way for the Queen's four-day stay, but there will be constant reminders of a violent past during her visit.   

The queen's arrival coincides with the 37th anniversary of bombings in Dublin and Monaghan, the single bloodiest day in a three-decade sectarian battle over Northern Ireland, and a day still mired in controversy.   

Relatives will lay a wreath at the site of one of the bombings in Dublin shortly before Queen Elizabeth and her husband, the Duke of Edinburgh, land at around 1100 GMT.

The monarch's visit is about reconciliation and emphasising the strong bond, built on generations of Irish emigration, that existed between the two countries even before a 1998 deal brought peace to Northern Ireland.   

It is estimated that nearly 1 in 10 British people have an Irish grandparent, entitling them to citizenship. Irish people are avid followers of British soccer clubs and soap operas. The nation of around 4.5 million people is the biggest overseas market for British clothing, food and drink.   

The queen won't, however, have much chance to meet ordinary Irish people. There will be no public walkabouts and onlookers will be limited to glimpses of the royal cavalcade, including an armoured Land Rover, as it whizzes through a locked-down Dublin.

The threat from militant nationalist groups opposed to the peace process in Northern Ireland necessitates the arm's length approach but also removes any potential embarrassment from a lack of large cheering crowds.

Most Irish people are impressed at the length and scope of the queen's visit and hope it will show their country, still reeling from its banking crisis, in a more positive light. But they would be unlikely to queue in large numbers to greet her.

Dissident nationalists have said the queen is not welcome on Irish soil. A coded bombing warning on Monday triggered a security alert in London including the closure of The Mall, the broad avenue leading to Buckingham Palace.   

Ireland is mounting its largest-ever security operation to guard the queen, using some 8,000 police and 2,000 soldiers. - Reuters