Greater Manchester Police confirmed children are among those killed and wounded in the blast at the Manchester Arena on Monday evening that is being treated as a "terrorist incident".
Manchester City and Manchester United, along with many other sporting clubs and organisations, have expressed their sympathy following the attack at the end of an Ariana Grande concert.
The death toll increased to 22 on Tuesday morning and UEFA president Aleksander Ceferin has sent his regards to all those affected.
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"I am deeply saddened by the horrible terrorist attack in Manchester last night," Ceferin said.
"It shocks me that so many innocent people lost their lives and I would like to send my condolences to the families of those affected."
United are playing in the UEFA Europa League final against Ajax on Wednesday evening in Stockholm, where five people died last month when a hijacked truck was deliberately driven into crowds.
A statement from European football's governing body in the wake of the Manchester attacks read: "There is currently no specific intelligence which might suggest that any of the UEFA Europa League Final activities in Stockholm may be the target of attacks.
"UEFA has been closely working with local authorities and the Swedish FA for many months and the terrorist risk had been taken into account since the very beginning of the project.
"Furthermore, a number of additional security measures were implemented following the attacks in Stockholm last April.
"Due to the tight security arrangements, UEFA urges fans to arrive at the stadium as early as possible, as detailed checks will be made at the entrances, resulting in potential delays in accessing the stadium."
United have yet to travel out to Sweden and their players have been quick to express their condolences on social media.
"Can't believe what happened last night," defender Daley Blind said. "My thoughts go out to all those affected. #PrayForManchester"
United's Ashley Young posted: "Absolutely shocked to hear the news this morning. Thoughts with all that lost loved ones & prayers for those being treated in hospitals"
Earlier winger Jesse Lingard said: "My thoughts and prayers are with everyone affected by last nights attack on this beautiful city. We will stand together in this dark hour ❤️"
City players past and present also posted their sympathy.
"Shocking events for this great city," Raheem Sterling said. "My thoughts and prayers are with the families."
City captain Vincent Kompany posted: "I would like to express my deepest condolences to the families of the victims. It's a sad day for the great city of Manchester. #Unity"
Greater Manchester Police have set up a help centre for anyone needing assistance at Gate 11 of City's Etihad Stadium.
There is no news yet on how United might pay tribute in the Europa League final, but England are wearing black armbands for the Tuesday's match against Guinea at the Under-20s World Cup.
United club captain Wayne Rooney tweeted: "Devastating news this morning. Thoughts and prayers are with all those affected."
Former England captain David Beckham, who left Manchester United in 2003, expressed his sadness following the attack.
The 42-year-old said on Instagram: "Heartbreaking news from Manchester. As a father & a human what has happened truly saddens me. My thoughts are with all of those that have been affected by this tragedy..."
Manchester City midfielder Yaya Toure and Formula One driver Lewis Hamilton also posted messages of support on Twitter.
Toure said: " Such terrible news in Manchester. Thoughts and prayers with all those affected."
Hamilton posted: "What is happening to this world when you're not even safe at a concert. A sad day, my prayers are with you Manchester #prayformanchester."