Police received a bomb threat for London from dissident Irish republicans Monday, on the eve of an historic trip by Queen Elizabeth II to Ireland and a week before US President Barack Obama arrives there. The warning heightened tensions after a spike in violence in Northern Ireland and a statement by a top paramilitary group that the British monarch was wanted for "war crimes." "A bomb threat warning has been received relating to central London today. The threat is not specific in relation to location or time," a Scotland Yard spokesman said. "We believe the threat is in connection with dissident republican terrorism." Police closed The Mall near Buckingham Palace for several hours on Monday but a search found nothing suspicious, while there was also a controlled explosion of a suitcase nearby. Specialist officers were lowered into a sewer on a winch during the investigation, an AFP journalist at the scene said. Coded warnings have been traditionally used by Irish republican groups. There has been a recent upsurge in republican violence in Northern Ireland, with a policeman killed by a bomb in April, but Scotland Yard said there had been no change in the official threat level. "The threat level from Irish-related terrorism has not increased and remains at substantial meaning that an attack is a strong possibility. The threat level was raised in September 2010 from moderate to substantial," a statement said. Police on Monday closed off The Mall near Buckingham Palace because of a security alert. "This is lower than the overall threat to the UK from international terrorism which remains at severe." The queen's four-day visit to Ireland is surrounded by a massive security operation amid threats from dissident republicans opposed to any British presence on Irish soil. Police on both sides of the Irish border have arrested several dissident republican suspects amid fears that such groups will attempt to hijack the queen's visit with an attack. The Real IRA paramilitary splinter group said the queen was wanted for "war crimes" and was "not wanted on Irish soil", threatening to ensure that she and "her cheerleaders get that message". It will be first state visit to Ireland by a British monarch since the republic gained independence in 1922. The last British monarch to visit was King George V a century ago. Security in Britain and Ireland was already tight as President Obama will visit Ireland next week before going on to visit Britain.