Security arrangements have been reviewed for the two-day event and organisers said revellers would show the world Birmingham's "true spirit of love, unity and support".
The festival's Carnival Parade in Victoria Square will began after the silence at midday.
Police have reassured people an on-going operation is in place.
Salman Abedi, a British-born Muslim from a Libyan family, blew himself up, killing 22 men, women and children, as people were leaving an Ariana Grande concert on Monday night.
A total of 66 people remain in hospital, with 23 in critical care.
Assistant Chief Constable Alex Murray, of Westâ€ˆMidlands Police, said he understood people may feel "nervous" but there was a full operation in place for the city.
"I appreciate that people planning to attend may feel a sense of nervousness - I would like to take this opportunity to give you some reassurance that a full policing operation is in place and is subject to ongoing reviews," he said.
"We will have a large number of police officers, specials and police staff on hand to keep the public safe - this isn't only for the Pride event, but also for anyone visiting the city to enjoy all it has to offer."
Lawrence Barton, festival director, said:â€ˆ"Following the tragic events in Manchester, we have been overwhelmed by all your messages of support - and are truly humbled that you are coming out in large numbers to show your love and pride this weekend.
"Birmingham is a great city, and like Manchester, we shall be showing the world our true spirit of love, unity, and support for one another whilst honouring the victims of Monday night's attack."
A minute's silence for the victims of the Manchester Arena attack has been held at Birmingham's Pride festival.