President Xi Jinping led condolences from world leaders when he contacted Queen Elizabeth II on Tuesday after the deadly explosion in Manchester that killed 22, in addition to the suicide bomber, and injured 59.

Flowers as a tribute for victims of Monday's suicide bombing at Manchester Arena in the English city of Manchester are seen in front of the British embassy in Berlin, Germany May 23, 2017.??[Photo/Agencies]

Xi expressed deep sadness over those who lost their lives and offered his sympathy to the relatives of people who were killed or injured.

At this difficult moment, the Chinese people firmly stand by the British people, Xi said.

Premier Li Keqiang also sent a message of condolence to British Prime Minister Theresa May on Tuesday.

May described the attack as Britain's worst-ever terror attack. The bomber died in the attack. Police said they had arrested a 23-year-old man in Manchester in connection with the bombing but gave no further details.

The suicide bomber, named by police as 22-year-old Salman Abedi, born in Manchester, triggered an improvised explosive device in the midst of a crowd of young pop fans and their parents at Manchester Arena, in the center of the city, as United States pop star Ariana Grande finished a sell-out show attended by many teenagers.

Manchester medical staff said 12 children under the age of 16 were among the 59 injured. The dead included an 8-year-old girl, Saffie Rose Roussos, a pupil at Tarleton Primary School, in Lancashire.

Greater Manchester Police Chief Constable Ian Hopkinson told reporters: "We believe it was one man, acting alone, who used an improvised explosive device. We believe he is amongst the dead."

Queen Elizabeth II sent her "deepest sympathy" to Manchester bombing victims, adding that the whole nation had been "shocked". The queen and members of the royal family observed a minute's silence at Buckingham Palace in memory of the 22 lives lost.

On Tuesday evening, thousands attended a vigil for the victims in Albert Square, in front of Manchester's imposing Victorian town hall building. Other vigils were being planned across the country.

May chaired a meeting of COBRA, the country's emergency committee, and later told reporters: "This attack stands out for its appalling, sickening cowardice. We struggle to understand the warped and twisted mind that would bring carnage to young children. Terrorists will never win – our country and our way of life will always prevail."

In London, police authorities said additional armed officers would be on the streets of the capital as a precaution.

In Manchester, hospitals swung into mass-casualty mode as medical staff from surrounding areas volunteered to help. Paramedics at the scene also treated a number of walking wounded, police said.

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

"The public should remain alert, but not alarmed. The great city of Manchester has been affected by terrorism before. Its spirit was not bowed; its community continues."

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

Islamic State claimed responsibility for the bombing, describing the perpetrator as a "soldier of the caliphate", but there was no evidence there was any direct link or prior knowledge among the terror group about the attack or the attacker.

May and fellow politicians agreed to suspend general election campaigning ahead of the June 8 vote.

The UK's official terror status level, as laid down by the intelligence services, is "severe", which means an attack is highly likely. The highest level is "critical", indicating an attack is expected.

Andy Burnham, the recently elected mayor of Greater Manchester, said it was "an act of evil. It has been a very dark night. Manchester people will come together. We will not let this divide us".

The BBC said a number of people and local hotels had offered temporary accommodation to concert goers, and taxi drivers were offering free transportation.

Manchester Arena can hold up to 18,000 people. It is regularly used to stage concerts and Grande, a 23-year-old reality TV star, is part-way through a UK tour. Her management said she was not injured in the blast, which came just after she finished her last song. She sent out a message on Twitter to say she was "broken" by the blast and the deaths.

"I have no words," she said.

Her fans are mainly young teens and pre-teen girls.

Pictures on social media and news websites showed young girls, some wrapped in blankets, others bloodied and bandaged, being helped by emergency medical teams.

Police closed off central Manchester and the main railway station, Manchester Victoria, which is right next to the arena, was closed and train services suspended.

Twenty-year-old Jess, who was at the concert with her sister, Hannah, 17, described to Manchester Evening News the moment she heard the huge explosion.

"We had just come through the main doors near the McDonalds and we'd stopped just a few meters away," she said. "I just heard this blast and knew straight away what it was. My sister was on the floor face-down. She had been thrown down, a few people had. I don't know how I wasn't."

Gary Walker, from Leeds, was with his wife in the foyer, waiting to pick up his two daughters who were at the concert.

He told BBC Radio 5 Live: "I was waiting for the kids to come out. We heard the last song, and quite a few people were flooding out and then, suddenly, there was a massive flash and then a bang and smoke. I felt a bit of pain in my foot and my leg. My wife said ‘I need to lie down'. I lay her down, she'd got a stomach wound and possibly a broken leg. I was about three meters from the actual explosion."

Zhang Yunbi in Beijing contributed to the story