"A viable explosive device was found on a bus yesterday evening in Maynooth," near Dublin, a spokesman said, adding that police had been tipped off by an anonymous call. The device was defused by the Irish army, he said.

The historic four-day trip, the first by a British monarch to the Republic of Ireland since it gained independence from London in 1922, is a landmark moment aimed at normalizing relations between the two neighbouring states.

The queen will visit amid a massive security lockdown after the threat of Irish republican terrorism resurfaced with a coded bomb threat in London.

Central Dublin was in a police clampdown amid Ireland's biggest-ever security operation, while dissident republicans opposed to the peace process and the British sovereign's visit made a coded bomb threat in London on Monday.

Police sealed off roads near the sovereign's Buckingham Palace residence and carried out a controlled explosion after the first coded warning outside Northern Ireland for 10 years.

AFP