By Shelby Sebens

PORTLAND, Ore. (Reuters) - Crowds counted down the minutes to midnight then lit up joints as smoking marijuana became legal in Oregon on Wednesday and the legalization movement spread to a fourth U.S. state.

Hundreds gathered on the Burnside Bridge in downtown Portland and smoked under the glow of a neon city sign, marking the moment that the law allowing recreational use, backed by voters in November, came into effect.

The legislation opens the way for shops to sell marijuana by next year, though a bill headed to the governor's desk would allow bans on retail sales in cities and counties where at least 55 percent of voters voted against legal weed.

Similar legalization initiatives that have ushered in retail pot shops are already in force in Washington state and Colorado, reflecting a shifting landscape for a drug that remains illegal under federal law. Alaska, which also voted to legalize marijuana, hopes for pot shops in 2016.

About half of U.S. states allow marijuana for medical use. The District of Columbia has voted to allow recreational-use marijuana but not retail shops, and a pot legalization campaign is getting underway in California, where voters in 2010 rejected legal weed. Maine lawmakers blocked pot-legalization last month.

"We are thrilled with the end of adult marijuana prohibition, but we are far from where we need to be," said Russ Belville, from the pro-marijuana group NORML.

Oregon residents aged 21 and older can now smoke privately, grow up to four plants and possess up to eight ounces (227 grams) at home and one ounce outside home, the Liquor Control Commission said.

Driving while high remains illegal and pot cannot be taken out of state, even to neighboring Washington, where retailing started last year, the commission said.

Public smoking also remains illegal, but there were no immediate reports of arrests.

Regulators will start accepting business license applications in January, with stores slated for next fall.

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Portland International Airport warned travelers that although police would not seize marijuana, passengers remained subject to the laws at their final destinations.

"Nervousness about marijuana sort of becoming normed in our society is widespread," said Republican state Senator Ted Ferrioli.

Kevin Sabet, president of anti-marijuana group Smart Approaches to Marijuana, said: "When most Oregonians realize this is about Big Marijuana making money, and they see child poisonings from THC gummy bears and candies go up, along with car accidents, they may have reservations."





(Reporting by Shelby Sebens and Steve Dipaola in Portland, Oregon; Editing by Curtis Skinner, Christian Plumb and Andrew Hay)