The Bell System announced plans yesterday to build the world's longest laser telecommunications system, a 611-mile telephone line from Washington to Cambridge, Mass., capable of carrying 80,000 conversations on lightwaves shot through strands of glass.

The first leg of the $79 million system, from Washington to New York City, is scheduled to open in 1983, said telephone company officials, who predicted large savings from the technological breakthrough.

The system will carry conversations along 144 glass fibers as fine as a human hair bundled into a narrow cable, approximately half an inch wide through a series of digital, on-and-off light pulses. The digital nature of the transmissions will make the fiber optics system more compatible with computers and other business equipment.

The system will be built by AT&T and seven other Bell System companies, including the C&P Telephone Co., if the Federal Communications Commission approves the plans. Robert W. Kleinert, president of the AT&T long lines department, said the project is expected to save the Bell System nearly $50 million in construction and operating costs by 1990.

All but 96 miles of the telephone line will be laid in existing conduits. The laser line will be used to augment existing equipment and provide for growth rather than to replace functioning equipment, telephone company officials said. Most of the cable will be buried.

According to C&P Vice President Carroll E. Morgan, work on the first leg of the cable from a switching station in Southeast to Silver Spring is expected to begin in March.

Morgan said C&P initially will invest approximately $1 million in the project -- a figure he said represents only about three-tenths percent of what the company expects to invest next year. Telephone Company officials said they expected to ask for no rate increases to cover the costs per se of the new system.

Kleinert said the Bell System is studying the feasibility of extending systems to the South and the West along other high-density routes. Also under exploration is whether laser systems might be used for future transoceanic cables. Among the desirable features of the cables for routes with large volumes of use is that a greater number of conversations can be carried in a smaller space.

Besides being smaller and better suited for transmitting computer data, fiber optic systems are less subject to weather and electromagnetic interference and can provide better-quality transmission, Klemart said.

"It's revolutionary. It's a tremendous development in the history of telecommunications," Kleinart said.

Kleinert said the system allows for 30 to 40 percent growth in volume along the route served. The cost per circuit mile for the new system works out to about $9 a mile compared to a cost of about $13 a mile for equipment now in use, he said.