A Montgomery County Circuit Court jury has ordered a 31-year-old real estate broker to pay $325,000 to his father, a Bethesda broker who had accused the son in a $4 million suit of luring away customers and employes to launch his own real estate firm.

After deliberating for 13 hours, the jury returned a verdict last week in favor of Joseph E. Robert, 53, who contended in the three-week trial that while his son, Joseph E. Robert Jr., 31, was president and director of the family's company--J. Robert Co. Inc.--he solicited customers and asked agents to join his own, separate business.

Joseph E. Robert, whose firm primarily markets condominium apartments for developers in Maryland, Virginia and Florida, said the legal dispute with his son had devastated their relationship.

"This has been a year and a half of pure hell," he said yesterday. Robert, who broke into tears at one point while testifying in the trial, described his son yesterday as "an astute young businessman." But, the father added, "Many of the things he's done, he didn't learn from me."

Joseph E. Robert Jr. said he would have no comment about the case.

Witnesses in the trial, held before Judge Stanley B. Frosh, said the younger Robert left his father's firm in May 1981 to found J. E. Robert Co. He took many of his father's best agents with him--along with nearly all of the firm's accounts. The father testified that the lost accounts represented $75 million of the $80 million worth of property the company was handling.

George R. Kucik, who represented the elder Robert, argued that before the son left, he had begun to sign up his father's clients.

The jury concluded with its verdict that the younger Robert had illegally solicited and acquired the business of the J. Robert Co.'s customers as well as the services of the company's employes "for his personal benefit." At the time, he was president and director of his father's firm. The son handled day-to-day operations while the father looked for new customers, according to testimony.

The jury also found that Robert Jr. "intentionally, willfully and maliciously" breached his contract with the J. Robert Co. when he used confidential information to benefit his own business and to solicit customers and employes of his father's firm.

In addition to $325,000 awarded to the J. Robert Co., the jury made a separate award of $1 to the father in a another issue involving Marina Towers, an Alexandria condominium complex that the son had taken away as a business account. The dispute between the two firms over that property was decided two weeks earlier when a jury in Alexandria Circuit Court awarded the J. Robert Co. $87,000. The $1 award represented the jury's consensus that the account was taken unfairly, Kucik said.

Attorney David Povich argued that his client, Joseph Robert Jr., did nothing against his father's interests until after he was no longer employed at the company.

Povich maintained that if Robert Jr. did solicit employes or customers it was after he had left the firm. At that point, his actions were within what the law considers fair competition, Povich argued.

"It was a painful case for both father and son," Kucik said yesterday.