WASHINGTON - The Food and Drug Administration now has the authority to regulate tobacco products, under a law President Obama signed yesterday in a Rose Garden ceremony.

With children onstage and among audience members, Obama said the new law would curtail the "constant, insidious" advertising that tobacco companies targeted to kids.

He pointed out that nearly 90 percent of smokers start before age 18.

"I know; I was one of those teenagers," Obama said in his speech. "I know how hard it is to break the habit once you've started."

Later, though, he did not respond to a reporter's question about how his struggle to stop smoking was going.

Among those attending was Rep. G. K. Butterfield, a Democrat from Wilson, N.C., where auctioneers used to rattle off leaf prices after harvest each fall. He represents one of the heaviest tobacco-farming districts in the nation.

"This has been a very difficult issue for me," Butterfield said later. "But when I take a step back and look at it objectively, there's no question we need to reduce smoking."

The legislation was decades in the making. It passed both houses of Congress with strong bipartisan support but over the objections of tobacco-state lawmakers such as Sen. Richard Burr (R., N.C.), who held up other Senate floor business for nearly two weeks through parliamentary maneuvers to stretch out the debate.

Under the law, companies no longer may sell products with sweet, fruity, or spicy characterizing flavors. Warning labels must cover half of each cigarette package. There can be no products labeled "light" or "low-tar." Advertising is banned near playgrounds and schools. Companies no longer may give away free samples of cigarettes.

The FDA will have the authority to regulate the chemical makeup of products. It may sharply reduce - though not eliminate - nicotine, the addictive chemical in cigarettes.