wo federal registrars, dispatched by the Justice Department to register voters in this rural county of cotton fields and catfish ponds, set up shop in the tiny post office here Thursday night and asked Tommie Lee Petty, 52, to raise his right hand.

"Do you solemnly swear to faithfully support the Constitution of the United States and the State of Mississippi, so help you God?" asked Neilan Burns, 56, a retired federal employe from Atlanta who had been deputized on short notice.

"Yes, sir," said the soft-spoken black farmhand, who has spent his life working the fields for white plantation owners.

"We weren't allowed to vote in Mississippi for a long time. Never been allowed here to do too much."

But Petty's son, Tommy Jr., and three other blacks became bona fide Mississippi voters on a hot, muggy night in the hometown of late civil rights leader Fannie Lou Hamer.

Local political organizers today began rounding up other unregistered blacks in five rural Mississippi counties targeted by the Justice Department because of possible voting discrimination.

Assistant Attorney General William Bradford Reynolds ordered the registrars to Sunflower, Madison, Leflore, Humphreys and Quitman counties Thursday after blacks complained of a host of obstacles to voting and registration nearly two decades after the Voting Rights Act was passed.

President Reagan has scheduled a visit to the state next week.

In 1967, Petty registered at the Sunflower County seat in Indianola, 27 miles away, but he never signed up in town and so was ineligible to vote in city elections under the state's dual registration law. He has never voted.

"Ain't never tried," he shrugged. "Figured it wouldn't do no good. They whites going to get who they want anyhow. A bunch of us didn't want Ronald Reagan, but he got elected."

Sunflower County has never elected a black county supervisor even though it is 65 percent black. "There is bitterness and apathy," black Indianola attorney Carver Randle said. "Blacks believe it's futile to challenge the system because they've seen a black majority for so many years and whites still in control."

Former senator James O. Eastland, segregationist symbol of Mississippi politics, retired to his Doddsville plantation here after bragging for years that he had a "special pocket" in his coat for civil rights legislation.

On the telephone with Reynolds, Randle asked him to move registrars into the black community, away from the Indianola post office, which is across the street from a prominent white church. On Sunday, the last day federal registrars will sign up voters, vans are scheduled to bus blacks to the post office as whites attend church. "Some blacks still feel very uncomfortable about doing something that has been historically controversial like registering to vote in front of white folks who employ them, treat their sicknesses, loan them money and deliver county services," he said. "I know it's hard to believe, but some black men still get off the sidewalk and tip their hat when a white person comes along."

Reynolds agreed to consider it. "This is still Mississippi," argued Randle. "We're fighting the fear of white intimidation as much as any other obstacles."

It is almost 60 miles round trip from Ruleville to the county seat of Indianola. "I work 10 hours a day and can't get off without losing a day's pay," said Willie Dixon, 33, an auto parts worker who took the oath with Petty. "I need to do that."

Black leaders say inconvenient registration hours, economic intimidation and shifting polling places remain hurdles to voting. At the same time, the annexation of white suburbs and the gerrymandering of district lines dilute the black vote.

Unlike some white clerks who are said to hold hard and fast to office hours and "type with one finger" to discourage black registration, Sunflower's normal voting registrar, Sammy Ely, took his registration books on the road in April. He showed federal examiners every courtesy today when they picked up a copy of the rolls.

"You won't find anyone who is going to knock you in the head," said Ely. "They realize you've been sent here. Won't be any antagonistic feelings towards you, unless they're drunk."

But the historic 1960s-style federal intervention here stung white county officials. "It's a bunch of junk," said Jack Harper, powerful chancery clerk for 38 years. "They could have hired taxis for the people who wanted to come register and saved the taxpayers a lot of money."