Gun-rights groups have already filed a lawsuit against Chicago's handgun ban -- after a similar ban in Washington, D.C., was struck down by the Supreme Court.

The National Rifle Association is planning to go to court and challenge San Francisco's ban.

The high court ruled that Americans can keep guns at home for self-defense. It's the first time the justices ever ruled on the meaning of gun rights under the Second Amendment. Download and read the entire Supreme Court decision here.

But the court also said the right to bear arms is not absolute. And the mayor of Philadelphia, Michael Nutter, said that's an "explicit statement of support" for cities that are creating "reasonable measures" targeting those who will carry weapons in order to do harm.

The San Francisco ban prevents people from carrying guns on county property -- including parks, schools and community centers. San Francisco Mayor Gavin Newsom said he wishes the justices who rejected the D.C. ban would first "spend a week in public housing."

Chicago Mayor Richard Daley predicted that if his city's ban is overturned, there will be more violence.

Supreme Court Shoots Down D.C. Gun Law

On Thursday, the Supreme Court said Americans have a right to own guns for self-defense and hunting.

The court's 5-4 ruling strikes down the District of Columbia's 32-year-old ban on handguns as incompatible with gun rights under the Second Amendment. The decision goes further than even the Bush administration wanted, but probably leaves most firearms laws intact.

The court had not conclusively interpreted the Second Amendment since its ratification in 1791. The amendment reads: "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed."

Justice Antonin Scalia writes in the majority opinion that the Constitution does not permit "the absolute prohibition of handguns held and used for self-defense in the home."

He said the ruling would not affect laws that ban felons and the mentally ill from owning guns or prohibit people from carrying guns in sensitive places like schools and government buildings.

The basic issue for the justices was whether the amendment protects an individual's right to own guns no matter what, or whether that right is somehow tied to service in a state militia.

The justices won't return to the bench until Oct. 6, the start of their next term.

Lawmakers and gun rights supporters quickly reacted to the Supreme Court's ruling.

Sen. Russ Feingold, D-Wis., called it a long overdue decision, saying he didn't think "the precedent has been seriously reaffirmed in decades."

But fellow Democrat Dianne Feinstein of California said "70 years of precedent has gone out the window" and that the people of the country will be less safe because of it.

Wayne LaPierre of the NRA called the decision "the opening salvo in a step-by-step process."

Libertarian presidential candidate Bob Barr was quick to praising the ruling.

"It will go down as one of the Supreme Court’s most important rulings on behalf of liberty," Barr said. "Today’s decision marks a new era for gun rights in America."