(Adds dropped word 'in' in headline)
* Police say 22 dead, at least 59 wounded
* Police say attacker died at the scene
* Witness says she heard "massive explosion"
* PM May says police treating it as terrorist attack
* Campaigning suspended ahead of UK's June 8 election
By Michael Holden and Andrew Yates
MANCHESTER, England, May 23 (Reuters) - At least 22 people, including some children, were killed and 59 wounded when a suicide bomber struck as thousands of fans streamed out of a concert by U.S. singer Ariana Grande in the English city of Manchester on Monday.
Prime Minister Theresa May said the incident was being treated as a terrorist attack, making it the deadliest militant assault in Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005.
Police said the attacker died after detonating explosives shortly after 10:33 pm (2133 GMT) at Manchester Arena, which has the capacity to hold 21,000 people. Children were among the dead, police said.
"We believe, at this stage, the attack last night was conducted by one man," Manchester Chief Constable Ian Hopkins told reporters. "The priority is to establish whether he was acting alone or as part of a network.
"The attacker... died at the arena. We believe the attacker was carrying an improvised explosive device which he detonated causing this atrocity."
A witness who attended the concert said she felt a huge blast as she was leaving the arena, followed by screaming and a rush by thousands of people trying to escape the building.
A video posted on Twitter showed fans, many of them young, screaming and running from the venue. Dozens of parents frantically searched for their children, posting photos and pleading for information on social media.
"We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming," concert-goer Catherine Macfarlane told Reuters.
"It was a huge explosion - you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out."
Ariana Grande, 23, later said on Twitter: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
May, who faces an election in two-and-a-half weeks, said her thoughts were with the victims and their families. She and Jeremy Corbyn, the leader of the opposition Labour Party, agreed to suspend campaigning ahead of the June 8 election.
"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack," May said in a statement. "All our thoughts are with the victims and the families of those who have been affected."
May is due to hold a crisis response meeting.
Chinese President Xi Jinping sent his condolences over the blast to Britain's Queen Elizabeth, Chinese state media reported.
There was no immediate claim of responsibility, but U.S. officials drew parallels to the coordinated attacks in November 2015 by Islamist militants on the Bataclan concert hall and other sites in Paris, which claimed about 130 lives.
Islamic State supporters took to social media to celebrate the blast and some encouraged similar attacks elsewhere.
Britain is on its second-highest alert level of "severe", meaning an attack by militants is considered highly likely.
British counter-terrorism police have said they are making on average an arrest every day in connection with suspected terrorism.
In March, a British-born convert to Islam ploughed a car into pedestrians on London's Westminster Bridge, killing four people before stabbing to death a police officer who was on the grounds of parliament. The man was shot dead at the scene.
In 2015, Pakistani student Abid Naseer was convicted in a U.S. court of conspiring with al Qaeda to blow up the Arndale shopping centre in the centre of Manchester in April 2009.
Manchester Arena, the largest indoor arena in Europe, opened in 1995 and is a popular concert and sporting venue.
Desperate parents and friends used social media to search for loved ones who attended Monday's concert while the wounded were being treated at six hospitals across Manchester.
"Everyone pls share this, my little sister Emma was at the Ari concert tonight in #Manchester and she isn't answering her phone, pls help me," said one message posted alongside a picture of a blonde girl with flowers in her hair.
Paula Robinson, 48, from West Dalton about 40 miles east of Manchester, said she was at the train station next to the arena with her husband when she felt the explosion and saw dozens of teenage girls screaming and running away from arena.
"We ran out," Robinson told Reuters. "It was literally seconds after the explosion. I got the teens to run with me."
Robinson took dozens of teenage girls to the nearby Holiday Inn Express hotel and tweeted out her phone number to worried parents, telling them to meet her there. She said her phone had not stopped ringing since her tweet.
"Parents were frantic running about trying to get to their children," she said. "There were lots of lots children at Holiday Inn."
Additional Reporting by Alistair Smout, Kate Holton and David Milliken in LONDON, Mark Hosenball in LOS ANGELES, John Walcott in WASHINGTON, D.C., Leela de Kretser in NEW YORK, Mostafa Hashem in CAIRO, and Ben Blanchard in BEIJING; Writing by Guy Faulconbridge and Nick Tattersall; Editing by Paul Tait and John Stonestreet