Long lines of cars formed Wednesday as Los Angeles gun owners turned in weapons for up to $200, in a gun buyback event brought forward after the Connecticut school shooting.

Authorities promised there would be no questions asked at the drive-thru style event, where owners handed over weapons including assault rifles and Uzis directly from their cars, in exchange for grocery store gift cards.

But a couple of pro-gun activists also turned out to protest against the Gun Buyback Program organized in traditionally liberal Los Angeles, trying to dissuade owners from giving in weapons to be destroyed.

Sergeant Rudy Lopez of the LA Police Department (LAPD) told AFP the event was going well compared to previous years.

"It's looking very positive. A lot of people are waiting an hour and a half (in line) and we have so far 420 handguns, rifles and shotguns," he said in Van Nuys, north of LA, one of two locations where weapons could be turned in.

"People want to do something to do their part to get more guns off the streets," he added, about halfway through the day.

The fact that police were asking no questions was a key incentive. "See that? That's a silencer," said Lopez, pointing an assault weapon. "That's illegal. We didn't say anything."

Weapons turned in also included TEC-9 semi-automatic handguns, as well as World War II rifles and vintage shotguns, one dating from 1895.

Separate trashcans were available for rifles, handguns, shotguns and ammunition and one for magazines. They were half-full by lunchtime. A table displayed at least 16 assault weapons.

Gun owners were given a $100 grocery store gift card in exchange for handguns, rifles and shotguns, or a $200 card for automatic weapons.

"Cities and states must join with the federal government to do everything we can, as quickly as we can, to keep our communities safe," said LA mayor Antonio Villaraigosa.

Villaraigosa brought forward the date of the event, usually held in May, following the December 14 shooting at an elementary school in Newtown, Connecticut, in which a gunman killed 26 people including 20 young children.

Critics question the effectiveness of gun buyback events, arguing that the weapons surrendered tend to be the least likely to be used in criminal activities, such as guns which are old or malfunctioning.

Lopez said police would check the weapons handed in, to make sure they are not reported as stolen or lost, and hand them back to their rightful owners if they are. All other weapons were due to be melted, he said.

In the line of cars, one 51-year-old man -- nearly all those returning weapons were male -- said he was turning in four rifles that belonged to his late father.

"The guns have been sitting around, and there's a money incentive, so why not?" he told AFP, declining to give his name.

Nearby at least two separate protesters voiced their disapproval buyback program.

"Get $$ for your gun ... We buy your Gun to donate it to a woman in danger. An armed woman will not be a victim," said one poster, advertising the website SonsofLibertyLA.com.

"Many countries have fallen into tyranny because people were unarmed," George Siegel, a 35-year-old merchant mariner told AFP.

Asked about criticism that the Connecticut shootings could have been partly due to America's comparatively liberal gun laws, he said: "That's very disrespectful. They are trying to play with people's emotions."