People employed by the Brighton Housing Trust, which helps people with housing problems from its offices in Commercial Road, are demonstrating over their pay and conditions.
Around 25 people work in the Eastbourne office, and some of those will be taking part in the rally in Brighton.
Unison branch secretary Alex Knutsen said union members had been forced into strike action by "an employer hell-bent on ripping up and imposing new contracts of employment".
"Staff will have working hours increased for less pay," said Alex Knutson. "For half of our members, there will be a pay cut of up to £6,000.
"This is not as bad as the original proposals to slash sick/maternity pay and other conditions, which Unison successfully fought off.
"Some of our members who work with homeless people will face the loss of their homes due to these proposals.
"Bluntly, BHT, once an employer of choice, has moved to one of the most heartless, aggressive and bullying organisations of the 80 the branch has members within.
"We want a suspension of the threat of dismissals and talks, to reach a negotiated settlement – nothing more, nothing less."
The demonstration begins in Brighton at 8am in London Road.