“Part of the strategy is, you sue everybody and you get people out bit by bit, and the whole enterprise sort of pays for itself,” Mr. Blumberg said.

It is the kind of behavior that housing activists say is common among landlords seeking to tap into the city’s heated housing market by forcing out rent-regulated tenants. And it comes as state lawmakers in Albany are negotiating rent-regulation laws that govern 1 million apartments in New York, which Mayor Bill de Blasio wants to strengthen.

Avi Schick, a lawyer representing the Orbach Group, defended the the landlord, calling the firm fair and committed to upgrading the buildings, many of which were in poor condition at the time they were purchased. “They inherited enormous problems and fixed them,” Mr. Schick said.

When the buildings were taken over, Mr. Schick said, many tenants were not using their apartments as their primary residence, a violation of state regulations and grounds for eviction.

The lawsuits, Mr. Schick said they involved mostly market-rate tenants and were primarily for chronic nonpayment of rent. “If tenants are paying their rent, as low as it might be, no one’s doing anything to them,” Mr. Schick said.

The Orbach Group’s actions have drawn the scrutiny of the New York State attorney general, Eric T. Schneiderman, though officials declined to provide details because they did not want to discuss a continuing investigation. But some tenants received a letter from Mr. Schneiderman’s office in January notifying them that the office was “investigating complaints of tenant harassment” in their buildings, including the use of private investigators, “frivolous eviction proceedings” and “aggressive, unwanted buyout offers to vacate rent-stabilized units.”