The letter came in the dead of winter, dated Jan. 29. In a written “notice of termination,” Toussaint Wortham’s landlord accused him of violating his lease by making some renovations in his kitchen, and telling him that he had two weeks to leave the rent-stabilized apartment he had lived in for more than 30 years.

A few weeks later, Mr. Wortham said, his rent check was returned; his landlord had refused to accept it. Then, he said, the building’s manager offered to buy him out of the unit if he agreed to vacate for good. After he refused, he was served with court papers.

Mr. Wortham, 39, contends that the new set of cabinets he installed more than five years earlier was not the impetus for his landlord’s action. The real problem was his rent; at $859 a month, it is far less than what the three-bedroom apartment would command at market rate in his increasingly popular Brooklyn neighborhood. Rather than call a lawyer, Mr. Wortham turned to a group called the Crown Heights Tenant Union.

Founded by a handful of young Occupy Wall Street veterans last summer, the Crown Heights Tenant Union is one of the most effective groups at dealing with issues raised by the rapid gentrification of the neighborhood, just east of Prospect Park. It has organized tenants in some 20 buildings, to combat what the group says are patterns of abuse directed at residents like Mr. Wortham, who live in the area’s many rent-regulated apartments.