Union members, including commission staff, will distribute leaflets outside the venue and use a scheduled question-and-answer session to highlight concerns.
They will claim the cuts will leave charities at greater risk of fraud and will urge management to be more vocal in speaking out against the reduced budget and the impact it will have on the commission’s ability to regulate charities.
The commission’s annual budget will fall from £29.3m last year to £21.3m in 2014/15.
Mark Serwotka, general secretary of the PCS, said: "Cuts to the Charity Commission are neither inevitable nor necessary, and will just heighten the risks of fraud and charities having to pay to receive expert advice and support.
"This means there will be less money for good causes, so not only will charities find it more difficult to stay afloat, the goodwill of the public will also be undercut."
The leaflet being distributed by the PCS, which has 300,000 members from the civil and public services, highlights a report this year by the National Fraud Authority, which says fraud costs charities £1.3bn a year.
The leaflets warn that fraud will increase and charities will have to pay more for advice and support as the regulator reduces its capacity.
A commission spokeswoman said its core role "will continue to be to protect the public interest in the integrity of charity".
She added: "This includes taking appropriate action to deal with fraud and malpractice, which we take extremely seriously.
"In addition, we work in partnership with law enforcement agencies at both strategic and operational levels on serious criminal issues, and will continue to do so."