LONDON - A major security alert was launched in London on Monday after Irish terrorists warned of a bomb near Buckingham Palace. Streets around The Mall, the road leads to the Buckingham Palace were closed for more than five hours, on the eve of the Queen's historic visit to Ireland. Bomb disposal officers carried out a controlled explosion on a suspect package outside a hotel yards from Trafalgar Square. Specialist officers searched sewers after police spotted a manhole cover in The Mall that had been disturbed. Scotland Yard sources confirmed a codeword was used in the warning. The threat was for a bomb to explode in London today, but the exact time or place was not specified. The terror threat status remained at "severe", the second highest level. There have been fears that dissident republican groups might try to mount an attack on the mainland in advance of the Queen's visit, which begins tomorrow. The Mall is where Prince William drove new bride Catherine in his father's Aston Martin after their wedding last month. Police were called to reports of a break-in near the Foreign Secretary's offices. When they were there an officer noticed a manhole cover was not se-cured properly and raised the alarm. Officers said nothing had been found but urged people to stay vigilant. Roads around St James's and Buckingham Palace were shut, causing rush-hour disruption. Ambulances were standing by. The area near the Queen Victoria memorial outside Buckingham Palace was open to tourists. The traditional Changing the Guard did not take place. The Met said it was working closely with Transport and City of London police. Security expert Crispin Black said London could have been chosen as a possible target because Dublin was seen as "too difficult" to attack. He said the Queen's visit had caused anger among republican extremists: "From what I have seen going on in Dublin it is going to be quite difficult to do anything there - it has been locked down. Maybe they decided ... to do something in London to show they are still there." The Irish Republic is staging its biggest ever security operation for the first tour of Ireland by a British monarch in 100 years. All police leave has been cancelled and armed Met officers will be in the Queen's security cordon. A team of 120 Scotland Yard royal protection officers will join her. Dissidents have increased their activity. The Irish Army bomb squad has been called out to 84 incidents this year, including a pipe bomb in the city centre. Two dissidents, including a man with alleged links to the 1998 Omagh bombing, were caught on CCTV last month at Trinity College Dublin, which is on the royal itinerary. One was using a camcorder. On Thursday police held three Dublin men in connection with "dissident and criminal groups". The Queen will visit Croke Park in Ireland, the scene of a massacre by British troops in 1920, and attend a wreath-laying at the Garden of Remembrance, which honours those who died for Irish freedom. She will also pay tribute to the 49,400 Irish soldiers who died in the First World War.