Ronaldo had been due to promote SIXPAD training gear at a press conference at the InterContinental Hotel on Saturday.
In a statement, SIXPAD said: “Following the upgrading of the UK’s terror threat, it is with regret that we have had to postpone Saturday’s public appearances with Cristiano Ronaldo.
“Safety is of paramount importance and given the high profile and public nature of this event we’ve taken the tough decision to postpone all activity until a later date.”
Several sports have reviewed their planned events following the Government’s decision to raise the national terror threat level to critical.
Monday’s devastating bombing at the Manchester Arena killed 22 people and left many more injured.
Ronaldo has a special bond with Manchester after playing for six years there with Manchester United, paid tribute to the victims of the terror attack on Instagram.
He said: “So sad to hear the news from Manchester. My thoughts are with the families and friends of the victims. #prayformanchester.”