A nonprofit housing watchdog group has sued a group of Boston-area landlords and real estate brokers alleging discrimination against low-income tenants.

Housing Rights Initiative filed a lawsuit in Suffolk Superior Court Wednesday, alleging that tenants with Section 8 and other housing vouchers were told their vouchers wouldn't be accepted at available listings.

The group argued that the 20 brokers and landlords named in the suit broke the law, and it is asking a judge to require them to change their policies or practices that have led them to deny housing vouchers. The lawsuit also seeks unspecified damages.

The defendants haven't responded to the lawsuit yet.

“As the largest fair housing lawsuit by defendant size in Massachusetts history, this lawsuit sends a clear message to every landlord and broker in the state: if you are a real estate company that discriminates against families and children with housing vouchers, the question of whether you will be caught is not a matter of if, but when,” Housing Rights Initiative founder Aaron Carr said in a release.

Lawyers for Civil Rights and Handley Farah & Anderson PLLC are representing the nonprofit in the suit.

HRI said it conducted a yearlong investigation that included “testers” posing as Section 8 tenants to inquire about available listings. The lawsuit includes screenshots of conversations with landlords and brokers, in which they allegedly tell tenants they don't accept housing vouchers.

Massachusetts had around 90,000 households that relied on the federal voucher program as of November, 10,000 households that used the state's housing assistance program as of December 2022, and 475 people that received vouchers through the Alternative Housing Voucher Program as of January 2019, according to the suit.

Under the state's anti-discrimination statute, it is illegal for a landlord to refuse to rent to those holding housing vouchers. However, tenants using vouchers have frequently faced discrimination from landlords and brokers who don't want to deal with the slow and lengthy approval processes, GBH reported in October 2022.

In December, Housing and Livable Communities Secretary Edward Augustus announced changes to the program to help unlock opportunities for voucher users in “high-rent neighborhoods,” MassLive reported. This was paired with a $100K media campaign to teach people that discrimination against voucher-using renters is illegal.