"Although it is not the first time Manchester has suffered in this way, it is the worst attack the city has experienced, and the worst ever to hit the north of England", May said outside the prime minister's residence in London, before heading to Manchester.
At least 22 people were killed and 59 injured after a bomb exploded outside the Manchester Arena on Monday night, as concertgoers - many of them children and young people - were leaving an Ariana Grande show.
The identities of the victims were becoming known little by little. They included an eight-year-old girl, two teenage girls and a 28-year-old man. A Polish couple who had come to collect their daughters after the concert also died, Poland's foreign minister said. He was described by neighbors as a tall, thin young man who often wore traditional Islamic dress, but few said they knew him well.
- He is of Libyan descent, sources in Manchester's Libyan community told CNN.
Investigators hunted Tuesday for possible accomplices of suicide bomber Salman Abedi, whose identity was provided by British counter-terrorism officials to US authorities.
It has now emerged that the univeristy drop out is believed to have travelled to Libya, as recently as this week. Campaigning for the June 8 election, which opinion polls suggest May's Conservative Party is on track to win, has been suspended as a mark of respect. The Changing the Guard ceremony at Buckingham Palace has also been cancelled on Wednesday to redeploy police officers, the UK's ministry of defence said. The status means armed soldiers could be deployed instead of police at public events including sports matches.
"Our priority, along with the police counter-terrorism network and our security partners, is to continue to establish whether he was acting alone or working as part of a wider network", Manchester Police Chief Constable Ian Hopkins said.
Attacks in cities including Paris, Nice, Brussels, St Petersburg, Berlin and London have shocked Europeans already anxious over security challenges from mass immigration and pockets of domestic Islamic radicalism.
Earlier Tuesday, ISIS claimed responsibility for the attack, but offered no evidence to support its claim. Islamic State has repeatedly called for attacks as retaliation for Western involvement in the conflicts in Syria and Iraq.