More than 690,000 employees and retirees signed up for new dental or vision insurance during the inaugural open season, far exceeding the government’s expectations.

The Office of Personnel Management, which administers federal benefits, had expected about 250,000 people to enroll in the new plans. The high interest overwhelmed the Web site dedicated to processing enrollments and forced OPM to extend enrollment for two additional weeks.

“I am very pleased with the success of this year’s open season for dental and vision benefits,” OPM Director Linda Springer said in a statement. “It proves that federal employees are ready, willing and able to take charge of their health care needs.”

The new benefits took effect Dec. 31. They supplement the mostly meager dental and vision insurance provided through existing medical plans. Unlike medical insurance, where the government picks up about 71 percent of premiums, enrollees in the supplemental dental or vision plans must pay the full premium.