Virginia state Sen. Omer L. Hirst, a Fairfax County Democrat and dean of Northern Virginia's delegation ot the General Assembly, announced yesterday that he will retire next year at the end of his current term.

The decision by Hirst, 65, a soft-spoken millionaire who helped lead the fight against Virginia's "massive resistance" to school integration in the 1950s, shocked many area politicians and is certain to prompt a vigorous challenge by Fairfax Republicans to win his seat.

"I just think it's a terrible loss and I hope we can convince him to reverse his position," said state Sen. Clive L. DuVal II, (D-Fairfax.) "It's a terrible loss."

"I'm kind of stunned," said Del. Richard L. Saslaw, the Annandale real estate salesman seen by some party regulars as a front-running Democrat to succeed Hirst. "I think it's a loss to the area."

Hirst, who is third in seniority in the 40-member state Senate and chairs the influential Privileges and Elections Committee, said he was announcing his retirement early "in order that the wonderful people who have supported me over the years will have adequate time to select a competent successor."

"I don't like this business of the candidate having to go down to the wire to decide what to do," he said.

Hirst was first elected to the Virginia legislature in 1954 when he won a Fairfax seat in the 100-member House of Delegates. He was reelected to the House three times and then retired from politics. In 1964 he ran for the state Senate and won the scat representing the central Annandale portion of the county that he has held since.

Several Democrats said Hirst had discussed the possibility of retirement recently, but had given no indication of his decision.

"The first I hears of it was when I got a call from a reporter," said state Sen. Ldelard L. Brault, the Fairfax Democrat who would beocme Northern Virginia's senior state senator upon Hirst's retirement. There was littie speculation yesterday on which Demecrats would seek Hirst's seat, in addition to Saslaw, who is the only Democratic state dilegate in Hirst's senatorial district.

Saslaw, who is serving his second term in the House, declined to speculate. "I don't know yet," he said. "I'm just surprised. I thought he'd run again."

Among the Republicans most frequently mentioned as contenders for the seat are Del. Warren E. Barry of Springfield and former Fairfax delgate James Tate. Neither could be reached for comment.

Hirst, however, stressed that he was not retiring because he feared competition in the 1979 election. "I am not leaving out of any sense of political pressure from any direction whatsoever," he said.

The chairmanship of the Privileges and Elections Committee, which he holds because of his seniority, would give him a strong influence in the redistricting of legislative and congressional seats after the 1980 census.

Hirst is remembered for his opposition to the Byrd organization's poll tax and as a fighter for one-man-one-vote apportionment that gave Northern Virginia equal representation in the legislature. Before the legislature redistricted the state, some areas had six times the representation per person that the Virginia suburbs had.

Hirst's interest in transportation also made the Beltway -- Interstate 495 -- an early interstate project. He also was an organizer of the Metropolitan Washington Regional Conference, the predecessor of the Metropolitan Washington Council of Governments.

He also has been a strong supportyer of the state's higher education system, particularly of George Mason University, located in Fairfax County.

Hirst, the son of a struggling cattle farmer, built a fortune in Fairfax real estate by buying up land that today is valued at well over $25 million. It includes a portion of the Landmark Shopping Center and large tracts in McLean.