The Arlington County Board approved regulations early today allowing developers to convert half of the 1,686-unit Arlington Towers complex in Rosslyn into cooperative apartments, ending months of controversy involving the project.

The board approved most features of a compromise between tenants of the complex and the developers, Monument Associates, that was reached only yesterday. It will reserve 100 units for elderly, disabled or low-income residents for the next 10 years. A novel feature of the plan requires Arlington police to approve a security plan for the complex, which is being renamed River Place.

In recent years Arlington has fought unsuccessfully to stem the tide of condominium and cooperative conversions, which have claimed about 20 percent of the county's rental housing stock.

Tenants of Arlington Towers have vigorously opposed the conversion proposed for their complex, claiming it would force many residents to vacate their apartments and displace many elderly tenants. They also charged earlier that the developers failed to maintain adequate security.

River Place spokesmen said they have given discounts to residents who purchase units, provided more than $122,000 in relocation payments for tenants and increased security.

A recent county staff memorandum also said developers failed to provide adequate information on building conditions, cooperative fees and maintenance costs in their public offerings statements. The developers said the information was available in their sales office.

The developers said they have no plans to convert the remaining half of the project from rental units to cooperatives for the time being.