London police arrested a man in possession of a knife near Buckingham Palace Wednesday as the U.K. remains on heightened alert following a terrorist attack in Manchester that killed at least 22 people.

Authorities detained the man on The Mall moments before Queen Elizabeth II passed by in a car on her way to St Paul's Cathedral, to attend a service to mark the centenary of the Order of the British Empire with her husband Prince Philip, the U.K. newspaper The Telegraph reported. The person was not identified and it is unclear if charges were filed.

Police said the incident, is not believed to be terror-related and no injuries were reported, the outlet said. Photos showed officers surrounding the man, handcuffed while sitting on the ground, with one of them holding a container that appeared to contain a large knife.

Buckingham Palace, Man with Knife, Arrest

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Queen Elizabeth II, Prince Philip

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Several arrests have been made near Buckingham Palace, a high-profile target, over the years. In 2013, two men wielding knives were arrested outside the gates in two separate incidents. In 2015, many members of the Queen's Guard, the soldiers stationed prominently in front of royal residences, were moved from their traditional posts and given extra protection themselves amid fears they could be targeted in terror attacks.

Craziest Royal Real Estate, Buckingham Palace

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The attack in Manchester took place Monday and marked the deadliest terrorist incident in the U.K. since the 2005 London bombings. A suicide bomber, a 22-year-old Manchester-born man whose parents are Libyan refugees, killed 22 people, including children, as crowds left Manchester Arena following an Ariana Grande concert. ISIS later claimed responsibility.

Following the attack, Britain increased its security threat level to "critical" from "severe," while Prime Minister Theresa May announced that members of the armed forces would boost security at key sites. On Wednesday, several London tourist attractions were shut down. The Changing of the Guard ceremony outside Buckingham Palace was canceled.

However, the tourist areas of Kensington Palace, the home of Prince WilliamKate Middleton and Prince George, 3, Princess Charlotte, 2, as well as Prince Harry, remain open with increased security.

The royal family issued statements following the Manchester suicide bombing.

"Like everyone, Catherine, Harry and I are left shocked and saddened by the tragedy that unfolded in Manchester overnight," William said Tuesday. "Hundreds of friends, parents, children and partners are confronting unimaginable grief today, and we send our thoughts to them all. We also send our thanks to the people of Manchester for their display of strength, decency and community that is an example to the world."

"The whole nation has been shocked by the death and injury in Manchester last night of so many people, adults and children, who had just been enjoying a concert," the Queen said. "I know I speak for everyone in expressing my deepest sympathy to all who have been affected by this dreadful event and especially to the families and friends of those who have died or were injured. I want to thank all the members of the emergency services, who have responded with such professionalism and care. And I would like to express my admiration for the way the people of Manchester have responded, with humanity and compassion, to this act of barbarity."

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