It was fair and mild Wednesday when Patricia L. Barry, a legal secretary in downtown Atlanta, left her office for a restaurant at noontime to celebrate her 26th birthday with a friend.

At Margaret Mitchell Square on the edge of the Five Points financial district, a berserk Vietnam veteran from New Jersy walked up fired a pawn shop pistol at her, then aimed the gun at his own head and pulled the trigger again.

Barry and Raymond P. Bunting, 32, of Atlantic City, died a few minutes later at a public hospital, unknown to each other, victims of a near-classic "senseless crime."

This week's murder-suicide, in full view of hundreds of pedestrians and motorists, came two days after the Georgia State Patrol began pulling out troopers dispatched to Atlanta two months ago for the summer street-crime crisis.

The double deaths have renewed the uproar that began in this tourist-dependent, image-conscious city with the June 26 slaying of a Columbus, Ohio, physician during a robbery attempt on a downtown street while he attended a convention.

Atlantans have resumed keeping track of the city's homicides, which as of today had reached 187 for 1979, compared to 143 for all of 1978. Overall crime in Atlanta jumped by 29 percent and 24 percent in the first two quarters of this year.

Mayor Maynard Jackson said no additional number of troopers or city police would have prevented the Mitchell Square shootings.

He said he incident was in part the fault of the Georgia General Assembly's failure to pass "sensible hand-gun legislation."

It "occurred with a gun that was bought in a pawn shop . . . in a state that has no waiting period at the point of purchase and bought by a man convicted three times for aggravatedassault and once for trying to burn his own parents' home down," the mayor said.

The dead woman's employer, former governor Carl Sanders, blamed the mayor and a permissiveness he said the Jackson administration tolerates.

Sanders, an influential lawyer who was governor from 1963 to 1967, told Jackson in a letter released at a press conference attended by county prosecutors and state legislators: "The reason every kook, nut and crackpot in the country comes to Atlanta is that they feel they can get away with things here that they can't do elsewhere."

To some observers, the exchange between the moderately liberal ex-governor and the city's first black mayor had at least a touch of political coloring.

Before the killings, Jackson this week had indicated interest in running next year against Sen. Herman Talmadge. Sanders confessed he was interested in making a race for governor in 1982, his first foray back into political life since his defeat in the 1970 Democratic primary by a relatively unknown state senator named Jimmy Carter.

Sanders, a Talmadge supporter, recently gave the senator a $1,000 campaign contribution. He is regarded as part of the business and political leadership in the state that is attempting to dissuade would-be opponents of Talmadge from making the race.

Political concerns aside, the commercial impact of Atlanta's growing crime image worries the city's leaders. The chamber of Commerce called yesterday for a federal law regulating the public movement of persons with records like the assailant's at Margaret Mitchell Square. A coalition of downtown Atlanta businesses, Central Atlanta Progress Inc., urged state industrial developers to locate more blue-collar industry in the area to combat the unemployment blamed for the petty thieves and brown-bag drinkers found in increasing numbers on downtown streets.

Atlanta got some advice from Detroit Mayor Coleman Young, in town for cermonies at the Martin Luther King, Jr. center shortly after the shootings. "Four, five years ago, Detroit was known as 'murder city,' the crime capital of America, a dead city," Young said. "Don't let it bother you. We were there."

It does bother Jackson, however. In an unpublicized chew-out of editors at the city's two daily newspapers last week, the mayor criticized front-page stories saying Atlanta's crime rise was three times the national average.

"They're reading it in Birmingham. They're reading it in Houston," Jackson reportedly said, referring to a pair of Sun Belt cities regarded as Atlanta rivals.

The city, meanwhile, is stressing that the rise in crime has dropped to 8 percent in the third quarter of this year, compared to a double-digit increase in the first six months.

When Atlanta Constitution columnist Lewis Grizzard warned Thursday that the crime war is fought by "the drunks and the punks against the rest of us . . . and we're losing, goddammit," the column inspired 650 calls through the newspaper's switchboard before mid-morning, all but one favorable.

Though some downtowners dismiss the Mitchell Square deaths as freakish, others say they will change their traveling habits to avoid high-crime areas.

An 18-year-old criminal justice major at Georgia State University showed a reporter the four-inch tear gas gun he carries in his jeans pocket.

"Have a nice day," a downtown worker told a passerby, adding, "and be safe."