In London, police said extra armed officers would be on the streets as a precaution.

Eyewitnesses in Manchester spoke of bodies strewed on the ground, covered in blood.

Eyewitness Andy Holey, waiting to pick up his wife and daughter, said: "As I was waiting, an explosion went off and it threw me about 30 feet from one set of doors to the other set of doors.

"When I got up I saw bodies lying on the ground. My first thought was to go into the arena to try to find my family. It was definitely an explosion and it was some force. It happened near the box office at the entrance to the arena."

Manchester Arena can hold up to 18,000 people.

People take part in a vigil for the victims of an attack on concert goers at Manchester Arena, in Trafalgar Square, London, Britain, May 23, 2017. [Photo/Agencies]

UK Home Secretary Amber Rudd issued a statement saying: "This was a barbaric attack, deliberately targeting some of the most vulnerable in our society - young people and children out at a pop concert."

Yuan Tiecheng, founder and CEO of Riskon International, a security company based in Beijing, noted that although Britain has developed strong abilities in identifying and preventing potential attacks, it is faced with a new situation.

"As major terrorist or extremist groups are being impacted worldwide, terrorist activities are being decentralized, and the attacks in the future may be more like a constant presence that confronts ordinary people and may take place around the corner," Yuan said.

The United States is considering tightening security. The US Department of Homeland Security said the public may experience increased security at public events.

Hong Kong's Asia World Expo, where Ariana Grande is to hold a concert in September, said it would improve security at all concerts and events. Baggage inspection, metal detectors and search dogs are expected, Reuters reported.

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