ATLANTA, Georgia (CNN) -- Jubilation turned quickly to anger Monday after Georgia's top prosecutor vowed he would fight a judge's order that would have set free Genarlow Wilson, a Georgia man serving a 10-year prison sentence for a consensual sexual encounter he had as a teenager.

Wilson's attorney, B.J. Bernstein, said her client would not accept a proposed plea bargain that would get him out of prison because it would require Wilson to plead guilty to a felony.

A judge ruled Monday that his conduct should have been punished as a misdemeanor.

"We're going to keep fighting," Bernstein told reporters at a news conference at the state Capitol. (Watch Wilson's mom, lawyer's tearful reaction to the judge's ruling)

She was flanked by civil rights leaders and black lawmakers, who expressed anger at the decision by Georgia Attorney General Thurbert Baker to block Wilson's release by filing an appeal of the judge's ruling.

Joseph Lowery, former president of the Southern Christian Leadership Conference, called Baker's decision "unnecessary, unjust and unfounded."

"We don't understand the reasoning or rationale of the attorney general on this issue," Lowery said. (Watch Lowery call the top prosecutor's action absurd and unjust )

Wilson is black, as is Baker.

Law changed but Wilson not included

Wilson, 21, was convicted of aggravated child molestation for having consensual oral sex with a 15-year-old girl when he was 17, during a 2003 New Year's Eve party in Douglas County, just west of Atlanta, Georgia.

Wilson was also charged with raping a 17-year-old girl at the party -- who prosecutors maintained was too intoxicated to consent -- but jurors acquitted him on that charge.

Under the state law in effect at the time, Wilson received a mandatory 10-year sentence and has already served more than two years.

Partly as a result of Wilson's conviction, state legislators changed the law to make such consensual conduct between teenagers a misdemeanor, rather than a felony. But that change wasn't made retroactive, so it did not affect Wilson.

Monday, acting on a petition filed by his attorneys, Monroe County Superior Court Judge Thomas Wilson ruled that Genarlow Wilson's punishment was cruel and unusual and voided it on constitutional grounds. The judge reduced the sentence to one year and said Wilson should not be put on Georgia's sex offender registry, as the old law required.

Wilson's attorneys hoped to use the ruling to free him from state prison. But shortly after it was handed down, Baker announced he would appeal the decision, a move that keeps Wilson behind bars.

In a statement announcing his decision, Baker said he filed the appeal to resolve "clearly erroneous legal issues," charging that the judge did not have the authority "to reduce or modify the judgment of the trial court."

'The ultimate abuse of power'

Baker also said prosecutors in Douglas County had offered a plea deal that would reduce Wilson's sentence, possibly to time already served, and place him in a program for first-time offenders.

Once his sentence was complete, his conviction would be removed from his record, and he would be taken off the sex-offender registry, the attorney general said.

But Bernstein said Wilson would not accept the proposed deal because it would still require him to plead guilty to a felony with a 15-year sentence, which would hang over him until the sentence had been served.

"That's the ultimate abuse of power as a prosecutor," said Bernstein. "I want this child out. Enough already."

Bernstein said Wilson's attorneys are now trying to get him released on bond while Baker's appeal makes its way through the courts; but, so far, they have not been successful.

Wilson's plight has drawn national attention and pleas for his release, including from former President Jimmy Carter, a former Georgia governor, and even some of the jurors who convicted him.

But late last year, the Georgia Supreme Court, on a 4-3 vote, turned down Wilson's appeal, and both Douglas County District Attorney David McDade and Baker have opposed moves by his attorneys to have him released.

Legislation that would make the change in Georgia's child molestation law retroactive in order to free Wilson failed to win approval earlier this year.